Indicator, Circuit.

A galvanometer used to show when a circuit is active, and to give an approximate measurement of its strength. It is a less accurate and delicate form of instrument than the laboratory appliance.

Inductance.

The property of a circuit in virtue of which it exercises induction and develops lines of force. It is defined variously. As clear and satisfactory a definition as any is the following, due to Sumpner and Fleming: Inductance is the ratio between the total induction through a circuit to the current producing it. "Thus taking a simple helix of five turns carrying a current of two units, and a.s.suming that 1,000 lines of force pa.s.sed through the central turn, of which owing to leakage only 900 thread the next adjacent on each side, and again only 800 through the end turns, there would be 800 + 900 + 1000 + 900 + 800, or 4,400 linkages of lines with the wire, and this being with 2 units of current, there would be 2,200 linkages with unit current, and consequently the self-inductance of the helix would be 2,200 centimetres." (Kennelly.) Inductance, as regards its dimensions is usually reduced to a length, hence the last word of the preceding quotation.

The practical unit of inductance is termed the henry, from Prof. Joseph Henry; the secohm, or the quad or quadrant. The latter alludes to the quadrant of the earth, the value in length of the unit in question.

[Transcriber's note: (L (di/dt) = V). A current changing at the rate of one ampere per second through a one henry inductance produces one volt.

A sinusoidal current produces a voltage 90 degrees ahead of the current, a cosine (the derivative of sine is cosine). One volt across one henry causes the current to increase at one ampere per second.]

Induction, Coefficient of Self.

The coefficient of self-induction of a circuit is the quant.i.ty of induction pa.s.sing through it per unit current in it. If a given circuit is carrying a varying current it is producing a varying quant.i.ty of magnetic induction through itself. The quant.i.ty of induction through the circuit due to its current is generally proportional to its current. The quant.i.ty for unit current is the coefficient of self-induction.

(Emtage.)

Induction, Cross.

The induction of magnetic lines of force in a dynamo armature core by the current pa.s.sing around such armature. These lines in a symmetrical two pole machine are at right angles to the lines of force which would normally extend across the s.p.a.ce between the two magnet poles. The joint magnetizing effect of the field and of the cross induction produces a distorted field between the poles .

Synonym--Cross-magnetizing Effect.

299 STANDARD ELECTRICAL DICTIONARY.

Induction, Electro-magnetic.

The inter-reaction of electromagnetic lines of force with the production of currents thereby.

A current pa.s.sing through a conductor establishes around it a field of force representing a series of circular lines of force concentric with the axis of the conductor and perpendicular thereto. These lines of force have attributed to them, as a representative of their polarity, direction. This is of course purely conventional. If one is supposed to be looking at the end of a section of conductor, a.s.suming a current be pa.s.sing through it towards the observer, the lines of force will have a direction opposite to the motion of the hands of a watch. The idea of direction may be referred to a magnet. In it the lines of force are a.s.sumed to go from the north pole through the air or other surrounding dielectric to the south pole.

Two parallel wires having currents pa.s.sing through them in the same direction will attract each other. This is because the oppositely directed segments of lines of force between the conductors destroy each other, and the resultant of the two circles is an approximation to an ellipse. As lines of force tend to be as short as possible the conductors tend to approach each other to make the ellipse become of as small area as possible, in other words to become a circle.

If on the other hand the currents in the conductors are in opposite directions the segments of the lines of force between them will have similar directions, will, as it were, crowd the intervening ether and the wires will be repelled.

Fig. 200. ATTRACTION OF CONDUCTORS CARRYING SIMILAR CURRENTS.

By Amp?re's theory of magnetism, (see Magnetism, Amp?re's Theory of,) a magnet is a.s.sumed to be encircled by currents moving in the direction opposite to that of the hands of a watch as the observer faces the north pole. A magnet near a wire tends to place the Amp?rian currents parallel to the wire, and so that the portion of the Amp?rian currents nearest thereto will correspond in direction with the current in the wire.

300 STANDARD ELECTRICAL DICTIONARY.

This is the principle of the galvanometer. A number of methods of memoria technica have been proposed to remember it by.

Thus if we imagine a person swimming with the current and always facing the axis of the conductor, a magnetic needle held where the person is supposed to be will have its north pole deflected to the right hand of the person.

Fig. 201. REPULSION OF CONDUCTORS CARRYING OPPOSITE CURRENTS.

Again if we think of a corkscrew, which as it is turned screws itself along with the current, the motion of the handle shows the direction of the lines of force and the direction in which the north pole of a needle is deflected. This much is perhaps more properly electro-dynamics, but is necessary as a basis for the expression of induction.

If a current is varied in intensity in one conductor it will induce a temporary current in another conductor, part of which is parallel to the inducing current and which conductor is closed so as to form a circuit.

If the inducing current is decreased the induced current in the near and parallel portion of the other circuit will be of identical direction; if increased the induced current will be of opposite direction.

This is easiest figured by thinking of the lines of force surrounding the inducing conductor. If the current is decreased these can be imagined as receiving a twist or turn contrary to their normal direction, as thereby establishing a turn or twist in the ether surrounding the other wire corresponding in direction with the direction of the original lines of force, or what is the same thing, opposite in direction to the original twist. But we may a.s.sume that the establishment of such a disturbance causes a current, which must be governed in direction with the requirements of the new lines of force.

The same reasoning applies to the opposite case.

301 STANDARD ELECTRICAL DICTIONARY.

The general statement of a variable current acting on a neighboring circuit also applies to the approach or recession of an unvarying current, and to the cutting of lines of force by a conductor at right angles thereto. For it is evident that the case of a varying current is the case of a varying number of lines of force cutting or being cut by the neighboring conductor. As lines of force always imply a current, they always imply a direction of such current. The cutting of any lines of force by a closed conductor always implies a change of position with reference to all portions of such conductor and to the current and consequently an induced current or currents in one or the other direction in the moving conductor.

As the inducing of a current represents energy abstracted from that of the inducing circuit, the direction of the induced current is determined by (Lenz's Law) the rule that the new current will increase already existing resistances or develop new ones to the disturbance of the inducing field.

In saying that a conductor cutting lines of force at right angles to itself has a current induced in it, it must be understood that if not at right angles the right angle component of the direction of the wire acts in generating the current. The case resolves itself into the number of lines of force cut at any angle by the moving wire.

The lines of force may be produced by a magnet, permanent or electro.

This introduces no new element. The magnet may be referred, as regards direction of its lines of force, to its encircling currents, actual or Amp?rian, and the application of the laws just cited will cover all cases.

Induction, Coefficient of Mutual.

The coefficient of mutual induction of two circuits is the quant.i.ty of magnetic induction pa.s.sing through either of them per unit current in the other. (Emtage.) It is also defined as the work which must be done on either circuit, against the action of unit current in each, to take it away from its given position to an infinite distance from the other; and also as the work which would be done by either circuit on the other in consequence of unit current in each, as the other moves from an infinite distance to its given position with respect to the other conductor. It depends on the form, size, and relative position of the two circuits; and on the magnetic susceptibilities of neighboring substances.

The ether surrounding two circuits of intensity i' and i" must possess energy, expressible (Maxwell) as 1/2 L i2 + M i i + 1/2 N i12. It can be shown that M i i1 in any given position of the two circuits is numerically equal (1) to the mutual potential energy of the two circuits (2) to the number of lines of induction, which being due to A, pa.s.s from A through B, or equally being due to B, pa.s.s from B through A, and M is styled the coefficient of mutual induction. (Daniell.)

302 STANDARD ELECTRICAL DICTIONARY.

Induction, Electrostatic.

An electrostatic charge has always an opposite and bound charge. This may be so distributed as not to be distinguishable, in which case the charge is termed, incorrectly but conventionally, a free charge. But when a charge is produced an opposite and equal one always is formed, which is the bound charge. The region between the two charges and permeated by their lines of force, often curving out so as to embrace a volume of cross-sectional area larger than the mean facing area of the excited surfaces, is an electrostatic field of force. The establishing of an electrostatic field, and the production of a bound charge are electrostatic induction.

An insulated conductor brought into such a field suffers a redistribution of its electricity, or undergoes electrostatic induction.

The parts nearest respectively, the two loci of the original and the bound charges, are excited oppositely to such charges. The conductor presents two new bound charges, one referred to the original charge, the other to the first bound charge.

Induction, Horizontal.

In an iron or steel ship the induction exercised upon the compa.s.s needle by the horizontal members of the structure, such as deck-beams, when they are polarized by the earth's magnetic induction. This induction disappears four times in swinging a ship through a circle; deviation due to it is termed quadrantal deviation. (See Deviation, Quadrantal.)

Induction, Lateral.