[Ill.u.s.tration: FIG. 48.--SEARCH LIGHT ON MOUNT LOWE, CALIFORNIA.]

Brilliant as is the arc lamp, far more in evidence is the incandescent lamp. The little gla.s.s bulb with its tiny thread of light we find everywhere. Popular opinion and the decision of the courts accord this invention to Thomas A. Edison. The evolution of the incandescent lamp is, however, interesting, and may be briefly sketched as follows:

[Ill.u.s.tration: FIG. 49.--FIRST INCANDESCENT LAMP, BY PROFESSOR GROVE, 1840.]

[Ill.u.s.tration: FIG. 50.--STARR-KING LAMP.]

In 1845 there appeared in the _Philosophical Magazine_ a description of what was probably the first incandescent electric light. It was devised in 1840 by William Robert Grove, the inventor of the Grove battery, and is ill.u.s.trated in Fig. 49. It is stated that he experimented and read by it for hours. It was described as follows:

"A coil of platinum wire is attached to two copper wires, the lower parts of which, or those most distant from the platinum, are well varnished; these are fixed erect in a gla.s.s of distilled water, and another cylindrical gla.s.s, closed at the upper end, is inverted over them, so that its open mouth rests on the bottom of the former gla.s.s; the projecting ends of the copper wires are connected with a voltaic battery (two or three pairs of the nitric acid combination), and the ignited wire now gives a steady light. Instead of making the wires pa.s.s through the water, they may be fixed to metallic caps well luted to the necks of a gla.s.s globe."

In 1845 August King patented, in England, an incandescent lamp, having an unsealed platinum burner, and also a carbon in a vacuum. Mr. King acted as agent for an American inventor, Mr. Starr, and the lamp came to be known as the Starr-King lamp, shown in Fig. 50. The burner was a thin plate or pencil of carbon B, enclosed in a Torricellian vacuum at the end of an inverted barometer tube, and held between the terminals of the connecting wires leading to a battery. In 1859 Moses G. Farmer lighted his house at Salem, Ma.s.s., by a series of subdivided electric lights, which was the first private dwelling lighted by electricity, and probably the first ill.u.s.tration of the feasibility of subdividing the electric current through a number of electric lamps.

In 1877 William E. Sawyer applied for a United States patent for an electric engineering and lighting system, and in January, 1878, entered into a partnership with Albon Man, and the "Sawyer-Man" lamp, see Fig.

51, was produced. In this an incandescent rod of carbon was inclosed in an atmosphere of nitrogen. This marked the beginning of a period of great activity in this field, which finally resulted in the well known form of electric lamp shown in Fig. 52, which was patented by Edison, No. 223,898, January 27, 1880. The distinctive features of this lamp consisted in a bowed filament of carbon of very thin, thread-like character, which was made of paper or carbonized cellulose. This, when sealed in a vacuum, would not burn away, but would give the proper incandescence, and by its small transverse dimension and high resistance to the current, permitted a proper distribution of the electric current to a number of lamps, without a special regulator for each lamp; and which could also be made so cheaply that the lamp could be thrown away when the burner was finally broken. Edison's claim on this feature of the electric lamp was sharply contested in an interference in the Patent Office by Sawyer and Man, with the decisions alternating first in favor of one and then of the other, but which finally resulted in the grant of a patent to Sawyer and Man, on May 12, 1885. A struggle then began in the courts, which on October 4, 1892, terminated in a decision by the United States Court of Appeals (Edison Electric Light Company vs. United States Lighting Company), awarding the incandescent lamp to Edison.

[Ill.u.s.tration: FIG. 51.--SAWYER-MAN LAMP.]

[Ill.u.s.tration: FIG. 52.--EDISON'S ELECTRIC LAMP.

_A_--Exhausted globe. _B_--Carbon filament. _CC_--Wires sealed in gla.s.s.

_D_--Line of fusion of two parts of globe. _EF_--Insulating material.

_G_--Screw-threads. _HI_--Metal socket. _J_--Fixture arm _K_--Circuit controlling key.]

In the early demonstration given by Edison great disturbance was caused in the stock exchanges among the holders of gas shares, as the sensational reportings in the press seemed to indicate that gas was to be superseded entirely. This uneasiness on the London Stock Exchange amounted on October 11, 1878, to a veritable panic, but while the electric light has more than fulfilled the prophecy made for it in many directions, gas shares still continue to be good stocks.

[Ill.u.s.tration: FIG. 53.--ELECTRIC LIGHT CIRCUIT.]

[Ill.u.s.tration: FIG. 54.--EDISON'S THREE WIRE SYSTEM OF ELECTRIC LIGHT CIRCUITS.]

Closely allied to the practical use of the incandescent lamp is the method of supplying and regulating the current from the dynamo. Although the alternating current is used for arc light, only the continuous current can be used for the incandescent lights, and the relation of the dynamo and the incandescent lamps is shown in Fig. 53, in which L represents the lamps between the main conducting wires leading from the dynamo, which latter has the coils of the field magnets arranged in a shunt or branch circuit, in which is interposed a regulator R in the form of a resistance coil with movable switch lever, by which more or less of the current is allowed to flow through the field magnet coils to suit the work being done. In late years automatic regulators have been provided for accomplishing this result. In Fig. 54 is shown what is known as the Edison "three wire system," patented March 20, 1883, No.

274,290. In this two dynamos are used as at D D, and the three wires emerge from the dynamos, one from the negative pole of one dynamo, another from the positive pole of the other dynamo, and the third or middle one is connected to both the other poles (positive and negative), of the two dynamos. For purposes of ill.u.s.tration, this may be compared to a three-storied arrangement of current, the upper wire representing the third story, the middle wire the second story, and the bottom one the first story. The fall from either story to the next represents the working energy, but from the top wire to the bottom would be equal to a fall from the third story to the first. The purpose of this arrangement is to save expense in copper wire, for while three main wires are used instead of two, the aggregate weight of the wires (when the lamps are arranged as shown), may be made so much less than two heavy wires as to make a very great saving in copper.

The uses of the incandescent light are legion. Besides those which are of common observation it is used for lighting the interior of mines, caves, and the dark apartments of ships, and does not foul the air. It is also used by divers in submarine operations; in the formation of advertising signs, and in pyrotechnics, but perhaps one of the most extraordinary uses to which it has been put is in exploring the interior of the human stomach and other cavities of the body, a patent for which was granted to M. C. F. Nitze, No. 218,055, July 29, 1879.

When an electric lamp is arranged with the opposite ends of the carbon burner connected, one to the outgoing, the other to the incoming wires from a dynamo, so as to be bridged across, this arrangement is said to be "in multiple" or "in parallel," and the lamps bear the a.n.a.logy of horses drawing abreast, and when the opposite ends of the carbon burner are placed in a gap or break in either the outgoing or the incoming wire, the arrangement is said to be "in series," and the lamps bear the a.n.a.logy of horses in tandem.

Explanation of electric nomenclature can best be given by the a.n.a.logy in hydrostatics of a stream of water pa.s.sing in the hose pipe from a fire-engine. The "watt" indicates the sum total unit of electrical power for a definite period of time, and in the hose pipe would be represented by the effective force of a definite volume of water, pa.s.sing at a definite pressure, during a definite period of time. "Volt"

is a pressure unit of electro-motive force, and would be represented by the power of the engine. "Ampere" would be the quant.i.ty, or volume unit, or cross section of the hose pipe, and the "ohm" would be the unit of frictional resistance. The "watt" then would be the "volt" multiplied by the "ampere"; thus 500 watts would be 10 amperes at 50 volts, or 50 amperes at 10 volts. Low tension circuits, such as are used for incandescent lights, range from 100 to 240 volts and are harmless.

Trolley circuits are usually 500 volts, and will kill an animal, but are not necessarily fatal to man. High tension currents from 2,000 to 5,000 volts, such as are used for arc lights, are fatal.

Of all modern inventions, not one has advertised itself in such a spectacular way as the electric light. Those who have seen the magnificent electrical displays at the Chicago Fair, the electrical celebrations in New York, and the Omaha Exhibition, need no introduction to its marvelous splendors and beauties. In the annual report for 1898 of the Edison Electric Illuminating Company of New York, its statement shows that for that city alone the gross earnings were $2,898,021. There were 9,990 users of the electric light, 443,074 incandescent lamps, and 7,353 arc lights. It is estimated that the electric light stations and plants in the United States alone amount to $600,000,000. In the year 1899 a single manufacturing concern (The General Electric Company) received orders for 10,000,000 incandescent lamps, which is about one-half of the present annual production. Sixteen years ago the lamps were $1 each; to-day they can be bought for 18 cents.

What the future has in store for the further development of the electric light no one may dare predict. Already a different form or manifestation of electric light has been demonstrated, in which neither the electric arc nor the incandescent filament is used, but a peculiar glow is seen disa.s.sociated from a direct material habitation, and produced by currents of enormous frequency and high potential, in accordance with the patent to Tesla, No. 454,622, June 23, 1891. Other worthy inventors in this field are at work, and its development will be one of the interesting problems of the Twentieth Century.

CHAPTER VIII.

THE TELEPHONE.

PRELIMINARY SUGGESTIONS AND EXPERIMENTS OF BOURSEUL, REIS AND DRAWBAUGH--FIRST SPEAKING TELEPHONE BY PROF. BELL--DIFFERENCES BETWEEN REIS' AND BELL'S TELEPHONES--THE BLAKE TRANSMITTER-- BERLINER'S VARIATION OF RESISTANCE, AND ELECTRIC UNDULATIONS BY VARIATION OF PRESSURE--EDISON'S CARBON MICROPHONE--THE TELEPHONE EXCHANGE--STATISTICS.

???e (far), and f??? (sound), are the Greek roots from which the word telephone is derived. It has the significance of transmitting sound to distant points, and is a word antedating the present speaking telephone, although this fact is generally lost sight of in the dazzling brilliancy of this latter invention. In the effort to hear better, the American Indian was accustomed to place his ear to the ground. Children of former generations also made use of a toy known as the "lovers' telegraph"--a piece of string held under tension between the flexible bottoms of two tin boxes--which latter when spoken into transmitted through the string the vibrations from one box to the other, and made audible words spoken at a distance. These expedients simply made available the superior conductivity of the solid body over the air to transmit sound waves. The electro-magnetic telephone operates on an entirely different principle.

It is a marvelous creation of genius, and stands alone as the unique, superb, and unapproachable triumph of the Nineteenth Century. For subtilty of principle, impressiveness of action, and breadth of results, there is nothing comparable with it among mechanical agencies. In its wonderful function of placing one intelligent being in direct vocal and sympathetic communication with another a thousand miles away, its intangible and mysterious mode of action suggests to the imagination that unseen medium of prayer rising from the conscious human heart to its omniscient and responsive G.o.d. The telegraph and railroad had already brought all the peoples of the earth into intimate communication and made them close kin, but the telephone transformed them into the closer relationship of families, and the tiny wire, sentient and responsive with its unlimited burden of human thoughts and human feelings, forms one of the great vital cords in the solidarity of the human family.

It is a curious fact that many, and perhaps most, great inventions have been in the nature of accidental discoveries, the by-products of thought directed in another channel, and seeking other results, but the telephone does not belong to this cla.s.s. It is the logical and magnificent outcome of persistent thought and experiment in the direction of the electrical transmittal of speech. Prof. Bell had his objective point, and keeping this steadily in view, worked faithfully for the accomplishment of his object in producing a speaking telephone, until success crowned his work. He probably did not realize at first the full magnitude of the achievement, but looking at it from the end of the Nineteenth Century, he might well exclaim in the language of Horace: "_Exegi monumentum acre perennius_."

Prof. Bell's conception of the telephone dates back as far as 1874. His first United States patent, No. 174,465, was granted March 7, 1876, and his second January 30, 1877, No. 186,787. It is generally the fate of most inventions, even of a meritorious order, to languish for many years, and frequently through the whole term of the patent, before receiving full recognition and adoption by the public, but the meteoric brilliancy of this invention at its first public announcement astonished the ma.s.ses, and inspired the admiration of the savants of the world.

When exhibited at the Centennial Exhibition in Philadelphia, in 1876, it was spoken of by Sir William Thomson, and Prof. Henry, as the "greatest by far of all the marvels of the electric telegraph."

[Ill.u.s.tration: FIG. 55.--PHILIP REIS' TELEPHONE.]

It is always the fate of the author of any great invention to be compelled to defend himself against the claims of others. It is one of the failings of human nature to lay claim to that which somebody else has obtained, and is an old story which finds its first ill.u.s.tration in the squabbles of childhood. When a troop of prattling boys hunt b.u.t.terflies among the daisies, and some sharp-eyed youngster has captured a prize, there are always others of his mates to cry, "I saw it first," and men are but grown-up boys. So in the history of the telephone, Prof. Bell has found compet.i.tors for this honor, and it is astonishing to know how close some of these prior experimenters came to success without reaching it. In 1854 Bourseul, of Paris _suggested_ an electric telephone, and in 1861 Philip Reis _devised_ an electric telephone which would transmit musical tones. Daniel Drawbaugh, of Pennsylvania, is alleged to have made an electric telephone in 1867-1868, and his claims against the Bell interests were fought vigorously in the Patent Office, and in the courts, but without success.

Elisha Gray's claims perhaps came nearer to establishing for him a share in the honor of inventing the speaking telephone than any other, for he filed a caveat in the United States Patent Office upon the same day (February 14, 1876), upon which Prof. Bell's application for a patent was made. But in the contest in the Patent Office with Gray, Edison, Berliner, Richmond, Holcombe, Farmer, Dolbear, Volker, and others, it was decided that Prof. Bell was the first to make a practically effective speaking telephone, and this conclusion has been sustained by the courts. Reis was a poor German school teacher at Friedrichsdorf, and in 1860 he took a coil of wire, a knitting needle, the skin of a German sausage, the bung of a beer barrel, and a strip of platinum, and constructed the first electric telephone. A typical form of his transmitter, see Fig. 55, was a box covered with a vibrating membrane E, and provided with a mouth-piece at one side. A platinum strip F was attached to the membrane or vibrating diaphragm E, and a platinum pointed hammer G rested lightly on the platinum strip F. The hammer G and platinum strip F were connected to the opposite ends of a wire, which had in its circuit a battery and a receiver. Air vibrations in the nature of sound waves in the box caused the diaphragm E to vibrate, and a separating make-and-break contact between the platinum strip F and the platinum point of hammer G caused a series of separate and distinct broken impulses to traverse the battery circuit and be received upon the receiver, which latter consisted of an iron rod with a coil of wire around it. That Reis' transmitter did alternately make and break the circuit, seems clear from his own memoir. A translation from this memoir, taken from the annual report (Jahresberichte) of the Physical Society of Frankfurt am Main for 1860-1861, reads as follows:

"At the first condensation (of air vibrations) the hammer-shaped little wire _d_ (G in our ill.u.s.tration), will be pushed back. At the succeeding rarefaction it cannot follow the return vibration of the membrane, and the current going through the little strip (of platinum) remains interrupted so long as until the membrane driven by a new condensation presses the little strip against _d_ (the hammer G) once more. In this way each sound wave effects an opening and closing of the current."

[Ill.u.s.tration: FIG. 56.--PROF. BELL'S TELEPHONE, MARCH 7, 1876.]

Reis evidently did not know how to make the vibrations of his diaphragm translate themselves into exactly commensurate and correlated electric impulses of equal rapidity, range, and quality. If he had done this, he would have had a speaking telephone, but a make-and-break contact could never do it, and hence he in his later instruments attached to them a telegraphic key in order that the sending operator might communicate with the receiving operator. If Reis' telephone had been a speaking telephone, this would have been unnecessary. Furthermore, it is inconceivable how the intelligent, progressive, and scientific Germans could have failed to have given to a speaking telephone in 1860 the immediate honor and attention that it deserved. In America, the Bell speaking telephone, invented in 1876, was known all over the civilized world the same year. Reis' broken contact circuit would transmit musical tones, because musical tones vary chiefly in rapidity of vibration, rather than in range, or quality, and the chattering contacts of Reis'

telephone would transmit musical tones because said contracts could be adjusted to the practically uniform range of vibration. Prof. Bell, however, had made a special study of articulate speech, and knew that speech was not essentially musical, but was composed of an irregular and discordant medley of vowel and consonant sounds, whose vibrations varied not only in pitch or rapidity like musical tones, but also in the quality or kind of vibrations as to range and loudness. In his invention, therefore, he did not make and break the circuit as did Reis, through the contact points, but he used the more sensitive plan of a constantly closed circuit, and merely caused the current to undulate in it by a principle of magnetic induction. This principle was first discovered by Oersted, and developed into the well known fact that when a piece of iron is moved back and forth from the poles of an electro-magnet an induced current is made to oscillate in the helix of the electro-magnet. The difference between Reis' separating make-and-break circuit, and the Bell continuous but undulating current, might be ill.u.s.trated by the difference between the impulses delivered by the beating of the drum sticks on the head of a drum, on the one hand, and the alternate pulling and slackening of a kite cord, on the other.

In the successive impacts on the head of a drum there could not be so sensitive a transfer of motion to the lower head of the drum as there would be transferred to the kite by the movement of the hand holding the kite cord. Reis' plan resembled the broken drum beats, and Bell's the kite cord, which always preserved a certain amount of tension. Bell accomplished his object by the means shown in Figs. 56 and 57, in which Fig. 56 represents his first patent of March 7, 1876, and Fig. 57 his second patent of January 30, 1877. In both cases the current was a continuously closed one, and was not alternately made and broken as by the separating contacts of Reis. Prof. Bell caused the vocal air vibrations to undulate or oscillate the continuously closed circuit by the principle of magnetic induction as follows (see Fig. 56): He caused diaphragm _a_, when spoken against, to vibrate the armature _c_ in front of the electro-magnet _b_, but without touching it, and as the armature approached and receded from the electro-magnet it induced an undulating but never broken current in the helix of this electro-magnet and along the line to and through the helix of the electro-magnet _f_ at the distant receiver, and this undulating current, influencing the armature _h_, which touched the diaphragm _i_ but not the electro-magnet, produced in the attractive influence of the magnet on this armature and diaphragm, vibrations of the same rapidity, range, and quality as those vocal vibrations that acted upon the first diaphragm _a_. In other words, the sequence of transference was air vibrations in A, mechanical vibrations of diaphragm _a_, electrical undulations traversing the line, induced vibrations in armature _h_ and diaphragm _i_, and air vibrations again resolved back into sounds of articulate speech, the same as those spoken into A. It will be perceived that in the Bell telephone both transmitter and receiver were of identical construction. This is better shown in Fig. 57 of his later patent, in which the horizontal line below the electro-magnet on one side represents a metal transmitting diaphragm, and the horizontal line under the electro-magnet at the other side was the receiving diaphragm. Not only were the sounds thus reproduced, but as the circuit was continuous and never broken by any separating contacts, the extreme sensitiveness of the electric vibrations set up by magnetic induction was such that the discordant and irregular quality of the vibrations of articulate speech were transferred and reproduced with exact fidelity, as well as the musical tones, and this rendered the speaking telephone a success. In later telephones the current is actually transmitted through the contacting points, but this only became practicable after the carbon microphone transmitter was invented, in which the essential undulations of the electric current were produced in another way, _i. e._, by the application of the important discovery that the varying of the pressure on carbon, by vibration, varied its conductivity, and in this way produced the same result of undulating a current without breaking it.

This in no wise detracts from the value of the principle of the continuous undulating current discovered and employed by Prof. Bell, between which and the breaks of the hard platinum points of Reis there is a difference as wide as the difference between success and failure.

[Ill.u.s.tration: FIG. 57.--PROF. BELL'S TELEPHONE, JANUARY 30, 1877.]

The form in which Prof. Bell's telephone was placed before the public was not that shown in the patents, but it quickly a.s.sumed the well-known shape of an elongated cylinder forming a handle, with a flaring mouth-piece at one end. This development in form is credited to Dr.

Channing in 1877, and it is the familiar form to-day, whose internal construction is shown in Fig. 58. The handle is made of hard rubber, and the cap or mouth-piece, which is screwed thereon, is also of hard rubber. The diaphragm A, of thin ferrotype plate, is clamped at its edges between the cap, or mouth-piece, and the handle. The compound magnet B is composed of four thin flat bar magnets, arranged in pairs on opposite sides of the flat end of the soft iron pole piece _c_ at one end, and the soft iron s.p.a.cing piece _d_ at the other end, the magnets being clamped to these pieces with like poles all in one direction. The end of the pole piece _c_ extends to within 1/100 to 2/100 of an inch of the diaphragm, or as near as possible so that the diaphragm does not touch it when it vibrates. On the pole piece _c_ is placed a wooden spool on which is wound silk-covered wire (No. 34, Am. W. G.). This wire fills the spool, and its ends are soldered to two insulated wires which pa.s.s through a flexible rubber disc _f_ below the spool and extend respectively to the two binding posts at the opposite end of the handle.

The current pa.s.ses from one binding post and its connecting wire, through the wire on the spool, and thence to the other connecting wire and binding post. When used as a transmitter, vocal vibrations acting mechanically on the diaphragm A produce undulatory vibrations by magnetic induction in the spool of wire, which are transmitted to the other end of the line; and when used as a receiver, the undulatory vibrations from the remote end of the line produce mechanical vibrations in the diaphragm, which set up air vibrations that are reproductions of articulate sounds.

[Ill.u.s.tration: FIG. 58.--LONGITUDINAL SECTION OF BELL TELEPHONE.]

Although the Bell telephone is both a transmitter and receiver, in practice a more sensitive and better form of transmitter has taken its place. That most generally used and best known is the "Blake transmitter," which was brought out about 1880. This employs two important elements. The first is the carbon microphone, which is a means for producing the undulations in the current by the variations in pressure on carbon contacts, and the second is an induction coil operated by a local battery, whose primary circuit pa.s.ses through the contacts of the carbon microphone, and whose secondary circuit pa.s.ses over the line. These fundamental elements of the Blake transmitter were the inventions of Berliner and Edison, and were made in 1877. The broad idea of producing electric undulations by varying the pressure between electrodes by vocal vibrations, was a large bone of contention in the Patent Office between various inventors. An application for a patent for the same was filed in the Patent Office by Emile Berliner, June 4, 1877, which was contested in an interference by Gray, Edison, Richmond, Dolbear, Holcombe, Prof. Bell, and others. After fourteen years of litigation the patent was finally awarded to Berliner. The patent granted to him November 17, 1891, No. 463,569, is a valuable one, and has become the property of the American Bell Telephone Company. The application of a low resistance conductor (carbon) in a microphone was invented by Edison as early as 1877, but his patent, No. 474,230, did not issue until May 3, 1892, on account of the interference with Berliner on the broader principle.

[Ill.u.s.tration: FIG. 59.--BLAKE TRANSMITTER.]

[Ill.u.s.tration: FIG. 60.--DIAGRAM OF CIRCUITS IN BLAKE TRANSMITTER.]

The Blake transmitter takes its name from the inventor of its mechanical features, who has a.s.sembled in it the fundamental principles of Berliner and Edison in a sensitive and practical mechanical construction, covered by minor patents, dated November 29, 1881. It is the little box in the middle of the familiar telephone outfit into which the talking is done.

Its internal construction is shown in Fig. 59. To the rear of the door is secured the cast iron circular ring A, inside of which lies the Russia iron diaphragm B, cushioned at its edges with a rubber band. A circular seat a little larger than the diaphragm is formed in the iron ring, and on this seat the diaphragm rests. A short, thin metal plate attached to the ring A on the right hand side clamps the diaphragm in position by resting squarely on the rubber edge of the diaphragm. Its function is like that of a hinge, which allows the diaphragm to freely swing inward. A steel damping spring is secured to the ring at the opposite edge of the diaphragm, and has its free end provided with a rubber glove on which is cemented a thin piece of fluffy woolen material. The padded end of the damping spring rests against the diaphragm and prevents excessive vibration. The iron ring A has at its bottom a projection holding an adjusting screw, and to a similar top projection is attached by screws a bra.s.s spring, from which depends another casting C, supporting the microphone apparatus, which is best shown in the diagram, Fig. 60. In this diagram A is one terminal of the battery connected by wire S to the hinge H of the box. From the other leaf of the hinge the wire M pa.s.ses to K, where it is soldered to the upper end of a German silver spring I. At K this spring is clamped and insulated from the iron work by two pieces of hard rubber. On the lower end of the spring I is soldered a short piece of thick platinum wire, whose ends are rounded into heads, one of which bears against the diaphragm N, and the other against the carbon b.u.t.ton J. This b.u.t.ton is attached to a small bra.s.s weight, and is supported by a spring R, clamped at its upper end to the metal support T. This spring is surrounded its entire length by rubber tubing to deaden vibration. The transmitter is adjusted by screw O, which, acting upon casting T, brings the carbon b.u.t.ton, the platinum heads, and also the diaphragm N, against each other with a regulated pressure. The current pa.s.ses from the part K to the spring I, the platinum head, carbon b.u.t.ton J, and its supporting spring R, to metal casting T, and ring V, thence by wire L to the lower hinge G, by wire P to the primary of the induction coil, and thence by wire Y to binding post B, the two binding posts A B being the two battery terminals. The secondary wire E of the induction coil has its ends connected by wires X and W with the two binding posts C B, which are the line terminals, or one the line terminal and the other the ground connection. It will thus be seen that the primary current pa.s.ses through the transmitter, and the secondary traverses the line. The most familiar forms of the telephone are those seen in Figs. 61 and 62, but the ideal form is rigged in a cabinet or little room, which excludes all extraneous interfering sounds.

[Ill.u.s.tration: FIG. 61.--WALL TELEPHONE.]

[Ill.u.s.tration: FIG. 62.--DESK TELEPHONE.]