The Standard Electrical Dictionary Part 116

Synonym--Congress Ohm.

396 STANDARD ELECTRICAL DICTIONARY.

Fig. 252. THEORY OF OHMMETER.

Fig. 253. OHMMETER.

Ohmmeter.

An instrument for measuring directly the resistance of a conductor or of any part of a circuit through which a strong current is pa.s.sing. It is the invention of Prof. W. E. Ayrton.

It contains two fixed coils at right angles to each other acting on the same needle of soft iron. One coil is of thick wire and is placed in series with the resistance to be measured. The other is of very thin wire and is placed in parallel with the same resistance. One wire acts by the total current, the other by the potential difference between the ends of the resistance. The action on the soft iron needle is due to the ratio of potential difference to total currents, or to the resistance itself. By properly designing and proportioning the coils the angular deflections of the needle are made proportional to the resistance.

In use the thick wire may be kept permanently in circuit. On connecting the binding posts of the thin wire coil to any two parts of the circuit its resistance is at once given by the deflection of the needle.

When no current is pa.s.sing the needle rests in any position. A current in the thick coil brings it to zero. A current simultaneously pa.s.sing through the thin high resistance coil brings about the deflection.

The instrument is a commercial rather than a scientific one.

Ohm's Law.

The fundamental law expressing the relations between current, electro-motive force and resistance in an active electric circuit. It may be expressed thus:

(a) The current strength is equal to the electro-motive force divided by the resistance.

(b) The electro-motive force is equal to the current strength multiplied by the resistance.

(c) The resistance is equal to the electro-motive force divided by the current strength. All these are different forms of the same statement.

Algebraically the law is usually expressed thus, (a) C = E/R. It may also be expressed thus: (b) E = C*R and (c) R= E/C, in which R denotes resistance, C denotes current strength, and E denotes electro-motive force.

Ohm, True.

The true ohm is the resistance of a column of mercury 1 square millimeter in cross-sectional area, and 106.24 centimeters long. (See Ohm.)

Synonym-Rayleigh Ohm.

Oil Insulation.

Oil insulation has received several applications in electrical work. It has been proposed for use in underground conduits. These it was proposed to fill with oil after the insertion of the conductors, the latter properly wrapped with cotton or other covering. For induction coils it has been very successfully used. Its princ.i.p.al utility depends on the fact that it is liquid, so that if pierced by a spark it at once closes again. A solid insulator if pierced is permanently injured. It is also used in telegraph insulators (see Insulator, Liquid) to prevent surface leakage.

397 STANDARD ELECTRICAL DICTIONARY.

Olefiant Gas.

A compound gas; C2H4; composed of carbon, 24; hydrogen, 4; molecular weight, 28; specific gravity, .981.

It is a dielectric of about the resistance of air. Its specific inductive capacity at atmospheric pressure is 1.000722 (Boltzman.)

Synonym--Ethene; heavy carburetted hydrogen.

[Transcriber's note: Also called ethylene. A primary use is polyethylene plastic.]

Open. adj.

An electric circuit is said to be open when it is cut or broken so that no current can pa.s.s through it. The term may be recollected by thinking of a switch; when open no current can pa.s.s through it. The same adjective is applied to magnetic circuits, an air gap implying an open circuit.

Open Circuit Oscillation.

An oscillation of current in open circuit so that a spark discharge accompanies it. It is produced by electric resonance in a simple circle or loop of wire with ends placed near together but not touching, if the circuit is of such size that its period of oscillation corresponds with that of the inducing discharge. (See Resonance, Electric.) Its period depends entirely on the self-induction of the circuit.

Ordinate.

In a system of plane co-ordinates (see Co-ordinates), the distance of any point from the axis of abscissas measured parallel to the axis of ordinates.

Ordinates, Axis of.

The vertical axis in a system of co-ordinates, q. v.

Synonym--Axis of Y.

Organ, Electric.

An organ in which the air blast is admitted or excluded from the different pipes by electric mechanism.

The outlines of the system are a series of contacts worked by the keys and stops, which cause, when operated by the organist, a current to pa.s.s through electro-magnets, opening the valves of the different pipes. Thus the manual may be at any distance from the organ, and a number of organs may be worked upon the same manual. As many as five in a single cathedral are thus connected to a manual in the chancel.

Orientation of a Magnetic Needle.

The acquirement by a magnetic needle of its position of rest, with its magnetic axis in the magnetic meridian.

Origin of Co-ordinates.

In a system of linear co-ordinates the point of intersection of the axes; the point whose co-ordinates are both zero.

398 STANDARD ELECTRICAL DICTIONARY.

Oscillating Needle.

A small light bar magnet suspended by a filament and employed in determining the intensity of a magnetic field by the oscillations it completes in a given time after a given disturbance.

Oscillations, Electric.

In static electricity the sudden and very rapid alternations in the discharge of a static condenser. This discharge of the disruptive order seems a single one, but is really composed of a number of discharges alternating in direction and producing electro-magnetic ether waves, probably identical with light waves except that they are longer and far less rapid.