The Standard Electrical Dictionary / A Popular Dictionary of Words and Terms Used in the Practice of Electrical Engineering

by T. O'Conor Sloane

Fig. 154. PORTABLE ELECTROMETER.
The general construction is seen in Fig. 154. There the fixed disc D is
carried by insulating stem g1. The charging electrode is supported by an
insulating stem g2, and without contact with the box passes out of its
cover through a guard tube E, with cover, sometimes called umbrella, V.
The umbrella is to protect the apparatus from air currents. At m is the
sighting lens. H is a lead box packed with pumice stone, moistened with
oil of vitriol or concentrated sulphuric acid, to preserve the
atmosphere dry. Before use the acid is boiled with some ammonium
sulphate to expel any corrosive nitrogen oxides, which might corrode the
brass.
In use the upper disc is charged by its insulated electrode within the
tube E; the movable disc is charged if desired directly through the case
of the instrument. The upper disc is screwed up or down by the
micrometer head M, until the sighted position is reached. The readings
of the micrometer on the top of the case give the data for calculation.
225 STANDARD ELECTRICAL DICTIONARY.

Fig. 155. LIPPMAN'S CAPILLARY ELECTROMETER.
Electrometer, Capillary.
An electrometer for measuring potential difference by capillary action,
which latter is affected by electrostatic excitement. A tube A contains
mercury; its end drawn out to a fine aperture dips into a vessel B which
contains dilute sulphuric acid with mercury under it, as shown. Wires
running from the binding-posts a and b connect one with the mercury in
A, the other with that in B. The upper end of the tube A connects with a
thick rubber mercury reservoir T, and manometer H. The surface tension
of the mercury-acid film at the lower end of the tube A keeps all in
equilibrium. If now a potential difference is established between a and
b, as by connecting a battery thereto, the surface tension is increased
and the mercury rises in the tube B. By screwing down the compressing
clamp E, the mercury is brought back to its original position. The
microscope M is used to determine this position with accuracy. The
change in reading of the manometer gives the relation of change of
surface tension and therefore of potential. Each electrometer needs
special graduation or calibration, but is exceedingly sensitive and
accurate. It cannot be used for greater potential differences than .6
volt, but can measure .0006 volt. Its electrostatic capacity is so small
that it can indicate rapid changes. Another form indicates potential
difference by the movement of a drop of sulphuric acid in a horizontal
glass tube, otherwise filled with mercury, and whose ends lead into two
mercury cups or reservoirs. The pair of electrodes to be tested are
connected to the mercury vessels. The drop moves towards the negative
pole, and its movement for small potential differences (less than one
volt) is proportional to the electro-motive force or potential
difference.
226 STANDARD ELECTRICAL DICTIONARY.
Electrometer Gauge.
An absolute electrometer (see Electrometer, Absolute) forming an
attachment to a Thomson quadrant electrometer. It is used to test the
potential of the flat needle connected with the inner surface of the
Leyden jar condenser of the apparatus. This it does by measuring the
attraction between itself and an attracting disc, the latter connected
by a conductor with the interior of the jar.
Electrometer, Lane's.
A Leyden jar with mounted discharger, so that when charged to a certain
point it discharges itself. It is connected with one coating of any jar
whose charge is to be measured, which jar is then charged by the other
coating. As the jar under trial becomes charged to a certain point the
electrometer jar discharges itself, and the number of discharges is the
measure of the charge of the other jar. It is really a unit jar, q. v.

Fig. 156. THOMSON'S QUADRANT ELECTROMETER.

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 glass 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 assumed
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 quantity 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 analogy, the
electro-motive force of the entire circuit might be expressed as 360º,
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 1º, for it would be measured
by the nearest path.
[Transcriber's notes: If 360º is the "long" way, 0º is the "short". A
formal restatement of the above definition of EMF: "If a charge Q passes
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 accumulating 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 passing 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 quantity 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 Potassium
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 Bismuth Antimony
Bismuth Bismuth Iron Lead
Antimony Nickel Copper Bismuth
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
passed 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 identity (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.
Electrophoric Action.
The action of an electrophorous; utilized in influence machines. (See
Electrophorous.)