The Standard Electrical Dictionary Part 84

Idiostatic Method.

A method of using the absolute or attracted disc electrometer. (See Electrometer, Absolute.) The suspended disc and guard ring are kept at the same potential, which is that of one of the points whose potential difference is to be determined; the lower fixed disc is connected to the other of the points whose potential difference is to be determined. Then we have the formula

V = d * SquareRoot( 8 * PI * F ) / S

in which d is the distance between the discs, V is the difference of potential of the two points, F the force of attraction between the discs in dynes, and S the area of the suspended disc. (See Heterostatic Method.)

296 STANDARD ELECTRICAL DICTIONARY.

Idle Poles.

Poles of wire sealed into Crookes' tubes, not used for the discharge connections, but for experimental connections to test the effect of different excitation on the discharge.

Idle Wire.

In a dynamo the wire which plays no part in generating electro-motive force. In a Gramme ring the wire on the inside of the ring is idle wire.

Igniter.

In arc lamps with fixed parallel carbons of the Jablochkoff type (see Candle, Jablochkoff) a strip of carbon connects the ends of the carbons in the unused candle. This is necessary to start the current. Such strip is called an igniter. It burns away in a very short time when an arc forms producing the light, and lasts, if all goes well, until the candle burns down to its end. Without the igniter the current would not start and no arc would form.

I. H. P.

Symbol for indicated horse-power.

Illuminating Power.

The relative light given by any source compared with a standard light, and stated in terms of the same, as a burner giving an illuminating power of sixteen candles. For standards see Candle, Carcel--Methven Standard--Pentane Standard.

Illuminating Power, Spherical.

The illuminating power of a lamp or source of light may vary in different directions, as in the case of a gas burner or incandescent lamp. The average illuminating power determined by photometric test or by calculation in all directions from the source of light is called the spherical illuminating power, or if stated in candles is called the spherical candle power.

Illumination, Unit of.

An absolute standard of light received by a surface. Preece proposed as such the light received from a standard candle (see Candle, Standard) at a distance of 12.7 inches. The object of selecting this distance was to make it equal to the Carcel Standard (see Carcel), which is the light given by a Carcel lamp at a distance of one meter.

From one-tenth to one-fiftieth this degree of illumination was found in gas-lighted streets by Preece, depending on the proximity of the gas lamps.

Image, Electric.

An electrified point or system of points on one side of a surface which would produce on the other side of that surface the same electrical action which the actual electrification of that surface really does produce. (Maxwell.)

The method of investigating the distribution of electricity by electric images is due to Sir William Thomson. The conception is purely a theoretical one, and is of mathematical value and interest.

297 STANDARD ELECTRICAL DICTIONARY.

Impedance.

The ratio of any impressed electro-motive force to the current which it produces in a conductor. For steady currents it is only the resistance.

For variable currents it may include besides resistance inductance and permittance. It is the sum of all factors opposing a current, both ohmic and spurious resistances. It is often determined and expressed as ohms.

Synonym--Apparent Resistance--Virtual Resistance.

Impedance, Oscillatory.

The counter-electro-motive force offered to an oscillatory discharge, as that of a Leyden jar. It varies with the frequency of the discharge current.

Synonym--Impulsive Impedance.

Impressed Electro-motive Force.

The electro-motive force expending itself in producing current induction in a neighboring circuit.

Impulse.

(a) An electro-magnetic impulse is the impulse produced upon the luminiferous ether by an oscillatory discharge or other varying type of current; the impulse is supposed to be identical, except as regards wave-length, with a light wave.

(b) An electro-motive impulse is the electro-motive force which rises so high as to produce an impulsive or oscillatory discharge, such as that of a Leyden jar.

Incandescence, Electric.

The heating or a conductor to red, or, more etymologically, to white heat by the pa.s.sage of an electric current. The practical conditions are a high intensity of current and a low degree of conductance of the conductor relatively speaking.

Inclination Map.

A map showing the locus of equal inclination or dips of the magnetic needle. The map shows a series of lines, each one of which follows the places at which the dip of the magnetic needle is identical. The map changes from year to year. (See Magnetic Elements.)

Independence of Currents in Parallel Circuits.

If a number of parallel circuits of comparatively high resistance are supplied by a single generator of comparatively low resistance, the current pa.s.sed through each one will be almost the same whether a single one or all are connected. Under the conditions named the currents are practically independent of each other.

[Transcriber's note: The current in each parallel branch depends on the resistance/impedance of that branch. Only if they all have the same impedance will the current be the same.]

Indicating Bell.

An electric bell arranged to drop a shutter or disclose in some other way a designating number or character when rung.

298 STANDARD ELECTRICAL DICTIONARY.

Indicator.

(a) An apparatus for indicating the condition of a distant element, such as the water level in a reservoir, the temperature of a drying room or cold storage room or any other datum. They are of the most varied constructions.

(b) The receiving instrument in a telegraph system is sometimes thus termed.