PART III.

MISCELLANEOUS INVENTIONS AND WRITINGS.

CHAPTER x.x.xIII.

METHOD OF OBTAINING DRIECT FROM ALTERNATING CURRENTS.

This method consists in obtaining direct from alternating currents, or in directing the waves of an alternating current so as to produce direct or substantially direct currents by developing or producing in the branches of a circuit including a source of alternating currents, either permanently or periodically, and by electric, electro-magnetic, or magnetic agencies, manifestations of energy, or what may be termed active resistances of opposite electrical character, whereby the currents or current waves of opposite sign will be diverted through different circuits, those of one sign pa.s.sing over one branch and those of opposite sign over the other.

We may consider herein only the case of a circuit divided into two paths, inasmuch as any further subdivision involves merely an extension of the general principle. Selecting, then, any circuit through which is flowing an alternating current, Mr. Tesla divides such circuit at any desired point into two branches or paths. In one of these paths he inserts some device to create an electromotive force counter to the waves or impulses of current of one sign and a similar device in the other branch which opposes the waves of opposite sign. a.s.sume, for example, that these devices are batteries, primary or secondary, or continuous current dynamo machines. The waves or impulses of opposite direction composing the main current have a natural tendency to divide between the two branches; but by reason of the opposite electrical character or effect of the two branches, one will offer an easy pa.s.sage to a current of a certain direction, while the other will offer a relatively high resistance to the pa.s.sage of the same current. The result of this disposition is, that the waves of current of one sign will, partly or wholly, pa.s.s over one of the paths or branches, while those of the opposite sign pa.s.s over the other. There may thus be obtained from an alternating current two or more direct currents without the employment of any commutator such as it has been heretofore regarded as necessary to use. The current in either branch may be used in the same way and for the same purposes as any other direct current--that is, it may be made to charge secondary batteries, energize electro-magnets, or for any other a.n.a.logous purpose.

Fig. 220 represents a plan of directing the alternating currents by means of devices purely electrical in character. Figs. 221, 222, 223, 224, 225, and 226 are diagrams ill.u.s.trative of other ways of carrying out the invention.

[Ill.u.s.tration: FIG. 220.]

In Fig. 220, A designates a generator of alternating currents, and B B the main or line circuit therefrom. At any given point in this circuit at or near which it is desired to obtain direct currents, the circuit B is divided into two paths or branches C D. In each of these branches is placed an electrical generator, which for the present we will a.s.sume produces direct or continuous currents. The direction of the current thus produced is opposite in one branch to that of the current in the other branch, or, considering the two branches as forming a closed circuit, the generators E F are connected up in series therein, one generator in each part or half of the circuit. The electromotive force of the current sources E and F may be equal to or higher or lower than the electromotive forces in the branches C D, or between the points X and Y of the circuit B B. If equal, it is evident that current waves of one sign will be opposed in one branch and a.s.sisted in the other to such an extent that all the waves of one sign will pa.s.s over one branch and those of opposite sign over the other. If, on the other hand, the electromotive force of the sources E F be lower than that between X and Y, the currents in both branches will be alternating, but the waves of one sign will preponderate. One of the generators or sources of current E or F may be dispensed with; but it is preferable to employ both, if they offer an appreciable resistance, as the two branches will be thereby better balanced. The translating or other devices to be acted upon by the current are designated by the letters G, and they are inserted in the branches C D in any desired manner; but in order to better preserve an even balance between the branches due regard should, of course, be had to the number and character of the devices.

[Ill.u.s.tration: FIG. 221.]

Figs. 221, 222, 223, and 224 ill.u.s.trate what may termed "electro-magnetic" devices for accomplishing a similar result--that is to say, instead of producing directly by a generator an electromotive force in each branch of the circuit, Mr. Tesla establishes a field or fields of force and leads the branches through the same in such manner that an active opposition of opposite effect or direction will be developed therein by the pa.s.sage, or tendency to pa.s.s, of the alternations of current. In Fig. 221, for example, A is the generator of alternating currents, B B the line circuit, and C D the branches over which the alternating currents are directed. In each branch is included the secondary of a transformer or induction coil, which, since they correspond in their functions to the batteries of the previous figure, are designated by the letters E F. The primaries H H' of the induction coils or transformers are connected either in parallel or series with a source of direct or continuous currents I, and the number of convolutions is so calculated for the strength of the current from I that the cores J J' will be saturated. The connections are such that the conditions in the two transformers are of opposite character--that is to say, the arrangement is such that a current wave or impulse corresponding in direction with that of the direct current in one primary, as H, is of opposite direction to that in the other primary H'. It thus results that while one secondary offers a resistance or opposition to the pa.s.sage through it of a wave of one sign, the other secondary similarly opposes a wave of opposite sign. In consequence, the waves of one sign will, to a greater or less extent, pa.s.s by way of one branch, while those of opposite sign in like manner pa.s.s over the other branch.

In lieu of saturating the primaries by a source of continuous current, we may include the primaries in the branches C D, respectively, and periodically short-circuit by any suitable mechanical devices--such as an ordinary revolving commutator--their secondaries. It will be understood, of course, that the rotation and action of the commutator must be in synchronism or in proper accord with the periods of the alternations in order to secure the desired results. Such a disposition is represented diagrammatically in Fig. 222. Corresponding to the previous figures, A is the generator of alternating currents, B B the line, and C D the two branches for the direct currents. In branch C are included two primary coils E E', and in branch D are two similar primaries F F' The corresponding secondaries for these coils and which are on the same subdivided cores J or J', are in circuits the terminals of which connect to opposite segments K K', and L L', respectively, of a commutator. Brushes b b bear upon the commutator and alternately short-circuit the plates K and K', and L and L', through a connection c. It is obvious that either the magnets and commutator, or the brushes, may revolve.

[Ill.u.s.tration: FIG. 222.]

The operation will be understood from a consideration of the effects of closing or short-circuiting the secondaries. For example, if at the instant when a given wave of current pa.s.ses, one set of secondaries be short-circuited, nearly all the current flows through the corresponding primaries; but the secondaries of the other branch being open-circuited, the self-induction in the primaries is highest, and hence little or no current will pa.s.s through that branch. If, as the current alternates, the secondaries of the two branches are alternately short-circuited, the result will be that the currents of one sign pa.s.s over one branch and those of the opposite sign over the other. The disadvantages of this arrangement, which would seem to result from the employment of sliding contacts, are in reality very slight, inasmuch as the electromotive force of the secondaries may be made exceedingly low, so that sparking at the brushes is avoided.

[Ill.u.s.tration: FIG. 223.]

Fig. 223 is a diagram, partly in section, of another plan of carrying out the invention. The circuit B in this case is divided, as before, and each branch includes the coils of both the fields and revolving armatures of two induction devices. The armatures O P are preferably mounted on the same shaft, and are adjusted relatively to one another in such manner that when the self-induction in one branch, as C, is maximum, in the other branch D it is minimum. The armatures are rotated in synchronism with the alternations from the source A. The winding or position of the armature coils is such that a current in a given direction pa.s.sed through both armatures would establish in one, poles similar to those in the adjacent poles of the field, and in the other, poles unlike the adjacent field poles, as indicated by n n s s in the diagram. If the like poles are presented, as shown in circuit D, the condition is that of a closed secondary upon a primary, or the position of least inductive resistance; hence a given alternation of current will pa.s.s mainly through D. A half revolution of the armatures produces an opposite effect and the succeeding current impulse pa.s.ses through C. Using this figure as an ill.u.s.tration, it is evident that the fields N M may be permanent magnets or independently excited and the armatures O P driven, as in the present case, so as to produce alternate currents, which will set up alternately impulses of opposite direction in the two branches D C, which in such case would include the armature circuits and translating devices only.

In Fig. 224 a plan alternative with that shown in Fig. 222 is ill.u.s.trated. In the previous case ill.u.s.trated, each branch C and D contained one or more primary coils, the secondaries of which were periodically short circuited in synchronism with the alternations of current from the main source A, and for this purpose a commutator was employed. The latter may, however, be dispensed with and an armature with a closed coil subst.i.tuted.

[Ill.u.s.tration: FIG. 224.]

Referring to Fig. 224 in one of the branches, as C, are two coils M', wound on laminated cores, and in the other branches D are similar coils N'. A subdivided or laminated armature O', carrying a closed coil R', is rotatably supported between the coils M' N', as shown. In the position shown--that is, with the coil R' parallel with the convolutions of the primaries N' M'--practically the whole current will pa.s.s through branch D, because the self-induction in coils M' M' is maximum. If, therefore, the armature and coil be rotated at a proper speed relatively to the periods or alternations of the source A, the same results are obtained as in the case of Fig. 222.

Fig. 225 is an instance of what may be called, in distinction to the others, a "magnetic" means of securing the result. V and W are two strong permanent magnets provided with armatures V' W', respectively. The armatures are made of thin laminae of soft iron or steel, and the amount of magnetic metal which they contain is so calculated that they will be fully or nearly saturated by the magnets. Around the armatures are coils E F, contained, respectively, in the circuits C and D. The connections and electrical conditions in this case are similar to those in Fig. 221, except that the current source of I, Fig. 221, is dispensed with and the saturation of the core of coils E F obtained from the permanent magnets.

[Ill.u.s.tration: FIG. 225.]

The previous ill.u.s.trations have all shown the two branches or paths containing the translating or induction devices as in derivation one to the other; but this is not always necessary. For example, in Fig. 226, A is an alternating-current generator; B B, the line wires or circuit. At any given point in the circuit let us form two paths, as D D', and at another point two paths, as C C'. Either pair or group of paths is similar to the previous dispositions with the electrical source or induction device in one branch only, while the two groups taken together form the obvious equivalent of the cases in which an induction device or generator is included in both branches. In one of the paths, as D, are included the devices to be operated by the current. In the other branch, as D', is an induction device that opposes the current impulses of one direction and directs them through the branch D. So, also, in branch C are translating devices G, and in branch C' an induction device or its equivalent that diverts through C impulses of opposite direction to those diverted by the device in branch D'. The diagram shows a special form of induction device for this purpose. J J' are the cores, formed with pole-pieces, upon which are wound the coils M N. Between these pole-pieces are mounted at right angles to one another the magnetic armatures O P, preferably mounted on the same shaft and designed to be rotated in synchronism with the alternations of current. When one of the armatures is in line with the poles or in the position occupied by armature P, the magnetic circuit of the induction device is practically closed; hence there will be the greatest opposition to the pa.s.sage of a current through coils N N. The alternation will therefore pa.s.s by way of branch D. At the same time, the magnetic circuit of the other induction device being broken by the position of the armature O, there will be less opposition to the current in coils M, which will shunt the current from branch C. A reversal of the current being attended by a shifting of the armatures, the opposite effect is produced.

[Ill.u.s.tration: FIG. 226.]

Other modifications of these methods are possible, but need not be pointed out. In all these plans, it will be observed, there is developed in one or all of these branches of a circuit from a source of alternating currents, an active (as distinguished from a dead) resistance or opposition to the currents of one sign, for the purpose of diverting the currents of that sign through the other or another path, but permitting the currents of opposite sign to pa.s.s without substantial opposition.

Whether the division of the currents or waves of current of opposite sign be effected with absolute precision or not is immaterial, since it will be sufficient if the waves are only partially diverted or directed, for in such case the preponderating influence in each branch of the circuit of the waves of one sign secures the same practical results in many if not all respects as though the current were direct and continuous.

An alternating and a direct current have been combined so that the waves of one direction or sign were partially or wholly overcome by the direct current; but by this plan only one set of alternations are utilized, whereas by the system just described the entire current is rendered available. By obvious applications of this discovery Mr. Tesla is enabled to produce a self-exciting alternating dynamo, or to operate direct current meters on alternating-current circuits or to run various devices--such as arc lamps--by direct currents in the same circuit with incandescent lamps or other devices operated by alternating currents.

It will be observed that if an intermittent counter or opposing force be developed in the branches of the circuit and of higher electromotive force than that of the generator, an alternating current will result in each branch, with the waves of one sign preponderating, while a constantly or uniformly acting opposition in the branches of higher electromotive force than the generator would produce a pulsating current, which conditions would be, under some circ.u.mstances, the equivalent of those described.

CHAPTER x.x.xIV.

CONDENSERS WITH PLATES IN OIL.

[Ill.u.s.tration: FIG. 227.]

[Ill.u.s.tration: FIG. 228.]

In experimenting with currents of high frequency and high potential, Mr. Tesla has found that insulating materials such as gla.s.s, mica, and in general those bodies which possess the highest specific inductive capacity, are inferior as insulators in such devices when currents of the kind described are employed compared with those possessing high insulating power, together with a smaller specific inductive capacity; and he has also found that it is very desirable to exclude all gaseous matter from the apparatus, or any access of the same to the electrified surfaces, in order to prevent heating by molecular bombardment and the loss or injury consequent thereon. He has therefore devised a method to accomplish these results and produce highly efficient and reliable condensers, by using oil as the dielectric[11]. The plan admits of a particular construction of condenser, in which the distance between the plates is adjustable, and of which he takes advantage.

[11] Mr. Tesla's experiments, as the careful reader of his three lectures will perceive, have revealed a very important fact which is taken advantage of in this invention. Namely, he has shown that in a condenser a considerable amount of energy may be wasted, and the condenser may break down merely because gaseous matter is present between the surfaces. A number of experiments are described in the lectures, which bring out this fact forcibly and serve as a guide in the operation of high tension apparatus. But besides bearing upon this point, these experiments also throw a light upon investigations of a purely scientific nature and explain now the lack of harmony among the observations of various investigators. Mr. Tesla shows that in a fluid such as oil the losses are very small as compared with those incurred in a gas.

In the accompanying ill.u.s.trations, Fig. 227 is a section of a condenser constructed in accordance with this principle and having stationary plates; and Fig. 228 is a similar view of a condenser with adjustable plates.

Any suitable box or receptacle A may be used to contain the plates or armatures. These latter are designated by B and C and are connected, respectively, to terminals D and E, which pa.s.s out through the sides of the case. The plates ordinarily are separated by strips of porous insulating material F, which are used merely for the purpose of maintaining them in position. The s.p.a.ce within the can is filled with oil G. Such a condenser will prove highly efficient and will not become heated or permanently injured.

In many cases it is desirable to vary or adjust the capacity of a condenser, and this is provided for by securing the plates to adjustable supports--as, for example, to rods H--pa.s.sing through stuffing boxes K in the sides of case A and furnished with nuts L, the ends of the rods being threaded for engagement with the nuts.

It is well known that oils possess insulating properties, and it has been a common practice to interpose a body of oil between two conductors for purposes of insulation; but Mr. Tesla believes he has discovered peculiar properties in oils which render them very valuable in this particular form of device.

CHAPTER x.x.xV.

ELECTROLYTIC REGISTERING METER.

An ingenious form of electrolytic meter attributable to Mr. Tesla is one in which a conductor is immersed in a solution, so arranged that metal may be deposited from the solution or taken away in such a manner that the electrical resistance of the conductor is varied in a definite proportion to the strength of the current the energy of which is to be computed, whereby this variation in resistance serves as a measure of the energy and also may actuate registering mechanism, whenever the resistance rises above or falls below certain limits.

In carrying out this idea Mr. Tesla employs an electrolytic cell, through which extend two conductors parallel and in close proximity to each other. These conductors he connects in series through a resistance, but in such manner that there is an equal difference of potential between them throughout their entire extent. The free ends or terminals of the conductors are connected either in series in the circuit supplying the current to the lamps or other devices, or in parallel to a resistance in the circuit and in series with the current consuming devices. Under such circ.u.mstances a current pa.s.sing through the conductors establishes a difference of potential between them which is proportional to the strength of the current, in consequence of which there is a leakage of current from one conductor to the other across the solution. The strength of this leakage current is proportional to the difference of potential, and, therefore, in proportion to the strength of the current pa.s.sing through the conductors. Moreover, as there is a constant difference of potential between the two conductors throughout the entire extent that is exposed to the solution, the current density through such solution is the same at all corresponding points, and hence the deposit is uniform along the whole of one of the conductors, while the metal is taken away uniformly from the other. The resistance of one conductor is by this means diminished, while that of the other is increased, both in proportion to the strength of the current pa.s.sing through the conductors. From such variation in the resistance of either or both of the conductors forming the positive and negative electrodes of the cell, the current energy expended may be readily computed. Figs. 229 and 230 ill.u.s.trate two forms of such a meter.

[Ill.u.s.tration: FIG. 229.]

In Fig. 229 G designates a direct-current generator. L L are the conductors of the circuit extending therefrom. A is a tube of gla.s.s, the ends of which are sealed, as by means of insulating plugs or caps B B. C C' are two conductors extending through the tube A, their ends pa.s.sing out through the plugs B to terminals thereon. These conductors may be corrugated or formed in other proper ways to offer the desired electrical resistance. R is a resistance connected in series with the two conductors C C', which by their free terminals are connected up in circuit with one of the conductors L.

The method of using this device and computing by means thereof the energy of the current will be readily understood. First, the resistances of the two conductors C C', respectively, are accurately measured and noted. Then a known current is pa.s.sed through the instrument for a given time, and by a second measurement the increase and diminution of the resistances of the two conductors are respectively taken. From these data the constant is obtained--that is to say, for example, the increase of resistance of one conductor or the diminution of the resistance of the other per lamp hour. These two measurements evidently serve as a check, since the gain of one conductor should equal the loss of the other. A further check is afforded by measuring both wires in series with the resistance, in which case the resistance of the whole should remain constant.

[Ill.u.s.tration: FIG. 230.]

In Fig. 230 the conductors C C' are connected in parallel, the current device at X pa.s.sing in one branch first through a resistance R' and then through conductor C, while on the other branch it pa.s.ses first through conductor C', and then through resistance R". The resistances R' R" are equal, as also are the resistances of the conductors C C'. It is, moreover, preferable that the respective resistances of the conductors C C' should be a known and convenient fraction of the coils or resistances R' R". It will be observed that in the arrangement shown in Fig. 230 there is a constant potential difference between the two conductors C C' throughout their entire length.

It will be seen that in both cases ill.u.s.trated, the proportionality of the increase or decrease of resistance to the current strength will always be preserved, for what one conductor gains the other loses, and the resistances of the conductors C C' being small as compared with the resistances in series with them. It will be understood that after each measurement or registration of a given variation of resistance in one or both conductors, the direction of the current should be changed or the instrument reversed, so that the deposit will be taken from the conductor which has gained and added to that which has lost. This principle is capable of many modifications. For instance, since there is a section of the circuit--to wit, the conductor C or C'--that varies in resistance in proportion to the current strength, such variation may be utilized, as is done in many a.n.a.logous cases, to effect the operation of various automatic devices, such as registers. It is better, however, for the sake of simplicity to compute the energy by measurements of resistance.

The chief advantages of this arrangement are, first, that it is possible to read off directly the amount of the energy expended by means of a properly constructed ohm-meter and without resorting to weighing the deposit; secondly it is not necessary to employ shunts, for the whole of the current to be measured may be pa.s.sed through the instrument; third, the accuracy of the instrument and correctness of the indications are but slightly affected by changes in temperature. It is also said that such meters have the merit of superior economy and compactness, as well as of cheapness in construction. Electrolytic meters seem to need every auxiliary advantage to make them permanently popular and successful, no matter how much ingenuity may be shown in their design.

CHAPTER x.x.xVI.

THERMO-MAGNETIC MOTORS AND PYRO-MAGNETIC GENERATORS.

No electrical inventor of the present day dealing with the problems of light and power considers that he has done himself or his opportunities justice until he has attacked the subject of thermo-magnetism. As far back as the beginning of the seventeenth century it was shown by Dr. William Gilbert, the father of modern electricity, that a loadstone or iron bar when heated to redness loses its magnetism; and since that time the influence of heat on the magnetic metals has been investigated frequently, though not with any material or practical result.

For a man of Mr. Tesla's inventive ability, the problems in this field have naturally had no small fascination, and though he has but glanced at them, it is to be hoped he may find time to pursue the study deeper and further. For such as he, the investigation must undoubtedly bear fruit. Meanwhile he has worked out one or two operative devices worthy of note.[12] He obtains mechanical power by a reciprocating action resulting from the joint operations of heat, magnetism, and a spring or weight or other force--that is to say he subjects a body magnetized by induction or otherwise to the action of heat until the magnetism is sufficiently neutralized to allow a weight or spring to give motion to the body and lessen the action of the heat, so that the magnetism may be sufficiently restored to move the body in the opposite direction, and again subject the same to the demagnetizing power of the heat.

[12] It will, of course, be inferred from the nature of these devices that the vibration obtained in this manner is very slow owing to the inability of the iron to follow rapid changes in temperature. In an interview with Mr. Tesla on this subject, the compiler learned of an experiment which will interest students. A simple horseshoe magnet is taken and a piece of sheet iron bent in the form of an L is brought in contact with one of the poles and placed in such a position that it is kept in the attraction of the opposite pole delicately suspended. A spirit lamp is placed under the sheet iron piece and when the iron is heated to a certain temperature it is easily set in vibration oscillating as rapidly as 400 to 500 times a minute. The experiment is very easily performed and is interesting princ.i.p.ally on account of the very rapid rate of vibration.

Use is made of either an electro-magnet or a permanent magnet, and the heat is directed against a body that is magnetized by induction, rather than directly against a permanent magnet, thereby avoiding the loss of magnetism that might result in the permanent magnet by the action of heat. Mr. Tesla also provides for lessening the volume of the heat or for intercepting the same during that portion of the reciprocation in which the cooling action takes place.

In the diagrams are shown some of the numerous arrangements that may be made use of in carrying out this idea. In all of these figures the magnet-poles are marked N S, the armature A, the Bunsen burner or other source of heat H, the axis of motion M, and the spring or the equivalent thereof--namely, a weight--is marked W.

[Ill.u.s.tration: FIG. 232.]

[Ill.u.s.tration: FIG. 231.]

[Ill.u.s.tration: FIG. 233.]

In Fig. 231 the permanent magnet N is connected with a frame, F, supporting the axis M, from which the arm P hangs, and at the lower end of which the armature A is supported. The stops 2 and 3 limit the extent of motion, and the spring W tends to draw the armature A away from the magnet N. It will now be understood that the magnetism of N is sufficient to overcome the spring W and draw the armature A toward the magnet N. The heat acting upon the armature A neutralizes its induced magnetism sufficiently for the spring W to draw the armature A away from the magnet N and also from the heat at H. The armature now cools, and the attraction of the magnet N overcomes the spring W and draws the armature A back again above the burner H, so that the same is again heated and the operations are repeated. The reciprocating movements thus obtained are employed as a source of mechanical power in any desired manner. Usually a connecting-rod to a crank upon a fly-wheel shaft would be made use of, as indicated in Fig. 240.

[Ill.u.s.tration: FIG. 234.]

[Ill.u.s.tration: FIG. 236.]

[Ill.u.s.tration: FIG. 235.]

Fig. 232 represents the same parts as before described; but an electro-magnet is ill.u.s.trated in place of a permanent magnet. The operations, however, are the same.

In Fig. 233 are shown the same parts as in Figs. 231 and 232, but they are differently arranged. The armature A, instead of swinging, is stationary and held by arm P', and the core N S of the electro-magnet is made to swing within the helix Q, the core being suspended by the arm P from the pivot M. A shield, R, is connected with the magnet-core and swings with it, so that after the heat has demagnetized the armature A to such an extent that the spring W draws the core N S away from the armature A, the shield R comes between the flame H and armature A, thereby intercepting the action of the heat and allowing the armature to cool, so that the magnetism, again preponderating, causes the movement of the core N S toward the armature A and the removal of the shield R from above the flame, so that the heat again acts to lessen or neutralize the magnetism. A rotary or other movement may be obtained from this reciprocation.

Fig. 234 corresponds in every respect with Fig. 233, except that a permanent horseshoe-magnet, N S is represented as taking the place of the electro-magnet in Fig. 233.

In Fig. 235 is shown a helix, Q, with an armature adapted to swing toward or from the helix. In this case there may be a soft-iron core in the helix, or the armature may a.s.sume the form of a solenoid core, there being no permanent core within the helix.

[Ill.u.s.tration: FIG. 237.]

[Ill.u.s.tration: FIG. 238.]

[Ill.u.s.tration: FIG. 239.]

Fig. 236 is an end view, and Fig. 237 a plan view, ill.u.s.trating the method as applied to a swinging armature, A, and a stationary permanent magnet, N S. In this instance Mr. Tesla applies the heat to an auxiliary armature or keeper, T, which is adjacent to and preferably in direct contact with the magnet. This armature T, in the form of a plate of sheet-iron, extends across from one pole to the other and is of sufficient section to practically form a keeper for the magnet, so that when the armature T is cool nearly all the lines of force pa.s.s over the same and very little free magnetism is exhibited. Then the armature A, which swings freely on the pivots M in front of the poles N S, is very little attracted and the spring W pulls the same way from the poles into the position indicated in the diagram. The heat is directed upon the iron plate T at some distance from the magnet, so as to allow the magnet to keep comparatively cool. This heat is applied beneath the plate by means of the burners H, and there is a connection from the armature A or its pivot to the gas-c.o.c.k 6, or other device for regulating the heat. The heat acting upon the middle portion of the plate T, the magnetic conductivity of the heated portion is diminished or destroyed, and a great number of the lines of force are deflected over the armature A, which is now powerfully attracted and drawn into line, or nearly so, with the poles N S. In so doing the c.o.c.k 6 is nearly closed and the plate T cools, the lines of force are again deflected over the same, the attraction exerted upon the armature A is diminished, and the spring W pulls the same away from the magnet into the position shown by full lines, and the operations are repeated. The arrangement shown in Fig. 236 has the advantages that the magnet and armature are kept cool and the strength of the permanent magnet is better preserved, as the magnetic circuit is constantly closed.

In the plan view, Fig. 238, is shown a permanent magnet and keeper plate, T, similar to those in Figs. 236 and 237, with the burners H for the gas beneath the same; but the armature is pivoted at one end to one pole of the magnet and the other end swings toward and from the other pole of the magnet. The spring W acts against a lever arm that projects from the armature, and the supply of heat has to be partly cut off by a connection to the swinging armature, so as to lessen the heat acting upon the keeper plate when the armature A has been attracted.

[Ill.u.s.tration: FIG. 240.]

[Ill.u.s.tration: FIG. 241.]

Fig. 239 is similar to Fig. 238, except that the keeper T is not made use of and the armature itself swings into and out of the range of the intense action of the heat from the burner H. Fig. 240 is a diagram similar to Fig. 231, except that in place of using a spring and stops, the armature is shown as connected by a link, to the crank of a fly-wheel, so that the fly-wheel will be revolved as rapidly as the armature can be heated and cooled to the necessary extent. A spring may be used in addition, as in Fig. 231. In Fig. 241 the armatures A A are connected by a link, so that one will be heating while the other is cooling, and the attraction exerted to move the cooled armature is availed of to draw away the heated armature instead of using a spring.

Mr. Tesla has also devoted his attention to the development of a pyromagnetic generator of electricity[13] based upon the following laws: First, that electricity or electrical energy is developed in any conducting body by subjecting such body to a varying magnetic influence; and second, that the magnetic properties of iron or other magnetic substance may be partially or entirely destroyed or caused to disappear by raising it to a certain temperature, but restored and caused to reappear by again lowering its temperature to a certain degree. These laws may be applied in the production of electrical currents in many ways, the principle of which is in all cases the same, viz., to subject a conductor to a varying magnetic influence, producing such variations by the application of heat, or, more strictly speaking, by the application or action of a varying temperature upon the source of the magnetism. This principle of operation may be ill.u.s.trated by a simple experiment: Place end to end, and preferably in actual contact, a permanently magnetized steel bar and a strip or bar of soft iron. Around the end of the iron bar or plate wind a coil of insulated wire. Then apply to the iron between the coil and the steel bar a flame or other source of heat which will be capable of raising that portion of the iron to an orange red, or a temperature of about 600 centigrade. When this condition is reached, the iron somewhat suddenly loses its magnetic properties, if it be very thin, and the same effect is produced as though the iron had been moved away from the magnet or the heated section had been removed. This change of position, however, is accompanied by a shifting of the magnetic lines, or, in other words, by a variation in the magnetic influence to which the coil is exposed, and a current in the coil is the result. Then remove the flame or in any other way reduce the temperature of the iron. The lowering of its temperature is accompanied by a return of its magnetic properties, and another change of magnetic conditions occurs, accompanied by a current in an opposite direction in the coil. The same operation may be repeated indefinitely, the effect upon the coil being similar to that which would follow from moving the magnetized bar to and from the end of the iron bar or plate.

[13] The chief point to be noted is that Mr. Tesla attacked this problem in a way which was, from the standpoint of theory, and that of an engineer, far better than that from which some earlier trials in this direction started. The enlargement of these ideas will be found in Mr. Tesla's work on the pyromagnetic generator, treated in this chapter. The chief effort of the inventor was to economize the heat, which was accomplished by inclosing the iron in a source of heat well insulated, and by cooling the iron by means of steam, utilizing the steam over again. The construction also permits of more rapid magnetic changes per unit of time, meaning larger output.

The device ill.u.s.trated below is a means of obtaining this result, the features of novelty in the invention being, first, the employment of an artificial cooling device, and, second, inclosing the source of heat and that portion of the magnetic circuit exposed to the heat and artificially cooling the heated part.

These improvements are applicable generally to the generators constructed on the plan above described--that is to say, we may use an artificial cooling device in conjunction with a variable or varied or uniform source of heat.

[Ill.u.s.tration: FIG. 242.]

[Ill.u.s.tration: FIG. 243.]

Fig. 242 is a central vertical longitudinal section of the complete apparatus and Fig. 243 is a cross-section of the magnetic armature-core of the generator.

Let A represent a magnetized core or permanent magnet the poles of which are bridged by an armature-core composed of a casing or sh.e.l.l B inclosing a number of hollow iron tubes C. Around this core are wound the conductors E E', to form the coils in which the currents are developed. In the circuits of these coils are current-consuming devices, as F F'.

D is a furnace or closed fire-box, through which the central portion of the core B extends. Above the fire is a boiler K, containing water. The flue L from the fire-box may extend up through the boiler.