Fig. 157. HENLEY'S QUADRANT ELECTROSCOPE.

227 STANDARD ELECTRICAL DICTIONARY.

Electrometer, Quadrant.

(a) Sir William Thomson's electrometer, a simple form of which is shown in the cut, consists of four quadrants of metal placed horizontally; above these a broad flat aluminum needle hangs by a very fine wire, acting as torsional suspension. The quadrants are insulated from each other, but the opposite ones connect with each other by wires. The apparatus is adjusted so that, when the quadrants are in an unexcited condition the needle is at rest over one of the diametrical divisions between quadrants. The needle by its suspension wire is in communication with the interior of a Leyden jar which is charged. The whole is covered with a gla.s.s shade, and the air within is kept dry by a dish of concentrated sulphuric acid so that the jar retains its charge for a long time and keeps the needle at approximately a constant potential. If now two pairs of quadrants are excited with opposite electricities, as when connected with the opposite poles of an insulated galvanic cell, the needle is repelled by one pair and attracted by the other, and therefore rotates through an arc of greater or less extent. A small concave mirror is attached above the needle and its image is reflected on a graduated screen. This makes the smallest movement visible.

Sometimes the quadrants are double, forming almost a complete box, within which the needle moves.

(b) Henley's quadrant electrometer is for use on the prime conductor of an electric machine, for roughly indicating the relative potential thereof. It consists of a wooden standard attached perpendicularly to the conductor. Near one end is attached a semi-circular or quadrant arc of a circle graduated into degrees or angular divisions. An index, consisting of a straw with a pith-bell attached to its end hangs from the center of curvature of the arc. When the prime conductor is charged the index moves up over the scale and its extent of motion indicates the potential relatively.

When the "quadrant electrometer" is spoken of it may always be a.s.sumed that Sir William Thomson's instrument is alluded to. Henley's instrument is properly termed a quadrant electroscope. (See Electroscope.)

Electro-motive Force.

The cause which produces currents of electricity. In general it can be expressed in difference of potentials, although the term electro-motive force should be restricted to potential difference causing a current. It is often a sustained charging of the generator terminals whence the current is taken. Its dimensions are

(work done/the quant.i.ty of electricity involved),

or ( M * (L^2) /(T^2 ) ) / ((M^.5) * (L^.5)) = ( (M^.5) * (L^1.5) ) /(T^2)

The practical unit of electro-motive force is the volt, q. v. It is often expressed in abbreviated form, as E. M. D. P., or simply as D. P., i. e., potential difference.

Electro-motive force and potential difference are in many cases virtually identical, and distinctions drawn between them vary with different authors. If we consider a closed electric circuit carrying a current, a definite electro-motive force determined by Ohm's law from the resistance and current obtains in it. But if we attempt to define potential difference as proper to the circuit we may quite fail.

Potential difference in a circuit is the difference in potential between defined points of such circuit. But no points in a closed circuit can be found which differ in potential by an amount equal to the entire electro-motive force of the circuit. Potential difference is properly the measure of electro-motive force expended on the portion of a circuit between any given points. Electro-motive force of an entire circuit, as it is measured, as it were, between two consecutive points but around the long portion of the circuit, is not conceivable as merely potential difference. Taking the circle divided in to degrees as an a.n.a.logy, the electro-motive force of the entire circuit might be expressed as 360?, which are the degrees intervening between two consecutive points, measured the long way around the circle. But the potential difference between the same two points would be only 1?, for it would be measured by the nearest path.

[Transcriber's notes: If 360? is the "long" way, 0? is the "short". A formal restatement of the above definition of EMF: "If a charge Q pa.s.ses through a device and gains energy U, the net EMF for that device is the energy gained per unit charge, or U/Q. The unit of EMF is a volt, or newton-meter per coulomb."]

228 STANDARD ELECTRICAL DICTIONARY.

Electro-motive Force, Counter.

A current going through a circuit often has not only true or ohmic resistance to overcome, but meets an opposing E. M. F. This is termed counter-electro-motive force. It is often treated in calculations as resistance, and is termed spurious resistance. It may be a part of the impedance of a circuit.

In a primary battery hydrogen acc.u.mulating on the negative plate develops counter E. M. F. In the voltaic arc the differential heating of the two carbons does the same. The storage battery is changed by a current pa.s.sing in the opposite direction to its own natural current; the polarity of such a battery is counter E. M. F.

Electro-motive Force, Unit.

Unit electro-motive force is that which is created in a conductor moving through a magnetic field at such a rate as to cut one unit line of force per second. It is that which must be maintained in a circuit of unit resistance to maintain a current of unit quant.i.ty therein. It is that which must be maintained between the ends of a conductor in order that unit current may do unit work in a second.

Electro-motive Intensity.

The force acting upon a unit charge of electricity. The mean force is equal to the difference of potential between two points within the field situated one centimeter apart, such distance being measured along the lines of force. The term is due to J. Clerk Maxwell.

Electro-motive Series.

Arrangement of the metals and carbon in series with the most electro-positive at one end, and electronegative at the other end. The following are examples for different exciting liquids:

Dilute Sulphuric Dilute Hydrochloric Caustic Pota.s.sium Acid Acid. Potash. Sulphide.

Zinc Zinc Zinc Zinc Cadmium Cadmium Tin Copper Tin Tin Cadmium Cadmium Lead Lead Antimony Tin Iron Iron Lead Silver Nickel Copper Bis.m.u.th Antimony Bis.m.u.th Bis.m.u.th Iron Lead Antimony Nickel Copper Bis.m.u.th Copper Silver Nickel Nickel Silver Antimony Silver Iron Gold Platinum Carbon

In each series the upper metal is the positive, dissolved or attacked element.

229 STANDARD ELECTRICAL DICTIONARY.

Electro-motograph.

An invention of Thomas A. Edison. A cylinder of chalk, moistened with solution of caustic soda, is mounted so as to be rotated by a handle. A diaphragm has an arm connected to its center. This arm is pressed against the surface of the cylinder by a spring. When the cylinder is rotated, a constant tension is exerted on the diaphragm. If a current is pa.s.sed through the junction of arm and cylinder the electrolytic action alters the friction so as to change the stress upon the diaphragm.

If the current producing this effect is of the type produced by the human voice through a microphone the successive variations in strain upon the diaphragm will cause it to emit articulate sounds. These are produced directly by the movement of the cylinder, the electrolytic action being rather the regulating portion of the operation. Hence very loud sounds can be produced by it. This has given it the name of the loud- speaking telephone.

The same principle may be applied in other ways. But the practical application of the motograph is in the telephone described.

Fig. 158. ELECTRO-MOTOGRAPH TELEPHONE

Electro-motor.

This term is sometimes applied to a current generator, such as a voltaic battery.

Electro-muscular Excitation.

A term in medical electricity indicating the excitation of muscle as the effect of electric currents of any kind.

Electro-negative. adj.

Appertaining to negative electrification; thus of the elements oxygen is the most electro-negative, because if separated by electrolytic action from any combination, it will be charged with negative electricity.

230 STANDARD ELECTRICAL DICTIONARY.

Electro-optics.

The branch of natural science treating of the relations between light and electricity. Both are supposed to be phenomena of or due to the luminiferous ether. To it may be referred the following:

(a) Electro-magnetic Stress and Magnetic Rotary Polarization;

(b) Dielectric Strain; all of which may be referred to in this book;

(c) Change in the resistance of a conductor by changes in light to which it is exposed (see Selenium);

(d) The relation of the index of refraction of a dielectric to the dielectric constant (see Electro-magnetic Theory of Light);

(e) The ident.i.ty (approximate) of the velocity of light in centimeters and the relative values of the electrostatic and electro-magnet units of intensity, the latter being 30,000,000,000 times greater than the former, while the velocity of light is 30,000,000,000 centimeters per second.