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

by T. O'Conor Sloane

556 STANDARD ELECTRICAL DICTIONARY.
V.
(a) Symbol for velocity.
(b) Symbol or abbreviation for volume.
(c) Symbol or abbreviation for volt.
557 STANDARD ELECTRICAL DICTIONARY.
V. A.
Symbol or abbreviation for voltaic alternatives, q. v.
Vacuum.
A space destitute of any substance. The great pervading substance is in
general sense the atmosphere. It is the gaseous mixture which surrounds
and envelopes the earth and its inhabitants. It consists of a simple
mixture of oxygen, 1 part, nitrogen, 4 parts, with 4 to 6 volumes of
carbonic acid gas in 10,000 volumes of air, or about one cubic inch to
one cubic foot. It presses with a force of about 14.7 lbs. per square
inch under the influence of the force of gravity. The term vacuum in
practise refers to any space from which air has been removed. It may be
produced chemically. Air may be displaced by carbonic acid gas and the
latter may be absorbed by caustic alkali or other chemical. The air may
be expelled and the space may be filled with steam which is condensed to
produce the vacuum. Of course in all cases the space must be included in
an hermetically sealed vessel, such as the bulb of an incandescent lamp.
But the universal method of producing a vacuum is by air pumps. An
absolute vacuum means the entire absence of gas or air, something almost
impossible to produce. A high vacuum is sometimes understood to mean one
in which the path of the molecules is equal in length to the diameter of
the containing vessels, as in Crookes' Radiometer and other apparatus
for illustrating the radiant condition of matter. The air left after
exhaustion is termed residual air or residual atmosphere.
[Transcriber's note: Dry air is about .78 nitrogen, .21 oxygen, .01
argon, .00038 carbon dioxide, and trace amounts of other gases. Argon
was suspected by Henry Cavendish in 1785. It was discovered in 1894 by
Lord Rayleigh and Sir William Ramsay.]
Vacuum, Absolute.
A space free of all material substance. It is doubtful whether an
absolute vacuum has ever been produced.
Vacuum, High.
An approximate vacuum, so nearly perfect that the molecules of the
residual gas in their kinetic motions rarely collide, and beat back and
forth between the walls of the containing vessel, or between any solid
object contained in the vessel and the walls of the vessel. The gas in
such a vacuum is in the radiant or ultra-gaseous state. (See
Ultra-gaseous Matter.)
Vacuum, Low.
A vacuum inferior to a high vacuum; a vacuum in which the molecules
collide with each other and do not move directly from side to side of
the containing vessel.
Vacuum, Partial.
A space partially exhausted of air so as to contain less than an equal
volume of the surrounding atmosphere. It really should come below a low
vacuum, but is often treated as synonymous therewith.
Vacuum, Torricellian.
The vacuum existing above the mercurial column in a barometer tube. The
principle of this vacuum is applied in the Geissler and other air pumps.
(See Pump, Geissler--Pump, Sprengel--Pump, Swinburne.)
558 STANDARD ELECTRICAL DICTIONARY.
Valency.
The relative power of replacing hydrogen or combining therewith
possessed by different elements; the number of atomic bonds belonging to
any element. Thus oxygen has a twofold valency, is bivalent or is a
dyad, and combines with two atoms of hydrogen because the latter has a
unitary atomicity, is monovalent or is a monad.
Valve, Electrically Controlled.
A valve which is moved by or whose movements are regulated by
electricity.
In the block system of railroad signaling the semaphores are worked by
weights and pneumatic cylinders and pistons. The valves for admitting or
releasing the compressed air are operated by coil and plunger mechanism.
There are many other instances of the control of valves by the electric
current.
Vapor Globe.
A protecting glass globe surrounding an incandescent lamp, when the lamp
is to be used in an atmosphere of explosive vapor, as in mines or
similar places; or when in a place where it is exposed to dripping water
which would break the hot lamp bulb if it fell upon it.
Variable Period.
The period of adjustment when a current is started through a conductor
of some capacity. It is the period of duration of the variable state, q.
v., in a conductor. As indicated in the next definition in a cable of
high electrostatic capacity a variable period of nearly two minutes may
exist. This indicates the retardation in signaling to be anticipated in
cables and other lines of high capacity.
Variable State.
When an electric circuit is closed the current starts through the
conductor with its full strength from the point of closure, and advances
with a species of wave front so that some time elapses before it attains
its full strength in the most distant parts of the conductor, owing to
its having to charge the conductor to its full capacity at the given
potential. The state of the line while the current thus varies is called
the variable state.
A long telegraph line when a message is being transmitted may be always
in the variable state. The current at the receiving end may never attain
its full strength.
In the case of such a conductor as the Atlantic cable, 108 seconds would
be required for a current to attain 9/10 of its full strength at the
distant end, and but 1/5 second to attain 1/100 of its final value.
During the period of increase of current the variable state exists.
Variation of the Compass.
The declination of the magnetic needle. (See Elements, Magnetic.) As the
declination is subject to daily, annual and secular variations, it is
unfortunate that this term is synonymous with declination. Thus the
variation of the compass means its declination, while there is also the
variation of the declination and of other elements. The term variation
of the compass is more colloquial than the more definite expression
"declination," or "magnetic declination."
559 STANDARD ELECTRICAL DICTIONARY.
Variometer.
An apparatus used in determining the relative values of the horizontal
component of the earth's magnetic field in different places.
Varley's Condenser.
A static condenser whose conducting surfaces are platinum electrodes
immersed in dilute sulphuric acid. When the potential difference is
1/50th that of a Daniell's cell, two square inches of platinum have a
capacity equal to that of an air condenser whose plates have an area of
80,000,000 square inches, and separated 1/8th of an inch from each
other. As the E. M. F. increases the capacity also increases.
Varley's Resistances.
Variable resistances formed of discs of carbonized cloth, q. v., piled
up, and pressed together more or less firmly to vary the resistance as
desired.
Varnish.
A glossy transparent coating of the nature of paint, applied as a
protective, or ornamental coating to objects.
Varnish, Electric.
Alcoholic or etherial varnishes are the best for electrical apparatus.
They dry quickly and perfectly, and tend to form surfaces unfavorable to
the hygroscopic collection of water. Sealing wax dissolved in alcohol,
or shellac dissolved in the same solvent are used for electrical
apparatus, although the first is rather a lacquer than a varnish.
Etherial solution of gum-copal is used to agglomerate coils of wire. It
is well to bake varnished objects to harden the coating.
Varnish, Red.
A solution of sealing wax in 90 per cent. alcohol. It is best made thin
and applied in several coats, each coat being allowed to dry perfectly
before the next is applied. It is often seen on Leyden jars. It is a
protector from surface leakage.
Vat.
A vessel for chemical or other solutions. A depositing vat is one in
which a plating solution is worked, for the deposition of electroplate
upon articles immersed in the liquid, and electrolyzed by an electric
current.
Velocity.
The rate of motion of a body. It is usually expressed in distance
traversed per second of time. The absolute unit is one centimeter per
second or kine. The foot per second is very largely used also.
The dimensions of velocity are length (L) divided by time (T) or L/T.
Velocity, Angular.
Velocity in a circle defined by the unit angle, or the angle which
subtends a circular arc equal in length to itself. The radius of the
circle traversed by the moving body does not enter into this definition,
as the real velocity of the object is not stated. If its angular
velocity and the radius of the path it travels are given its actual
velocity can be deduced.
560 STANDARD ELECTRICAL DICTIONARY.
Velocity of Signaling.
The speed of transmission of electric signals is affected by the nature
of the line, as regards its static capacity, and by the delicacy of the
receiving instruments, which may need a more or less strong current to
be affected. Thus of an original current one per cent. may suffice to
operate a sensitive instrument. This might give almost the velocity of
light, while if the instrument would only respond to the full current
nearly two minutes (see Variable State) might be required for the
production of a signal.
Velocity Ratio.
A term applied to the ratios existing between the electrostatic and
electro-magnetic units. If we take as numerators the dimensions of the
different qualities in the electrostatic system, and their dimensions in
the electro-magnetic system as denominators, the fractions thus obtained
reduce to expressions containing only velocity or V in some form. Thus
if we divide the dimensions of the electrostatic quantity by the
dimensions of electro-magnetic quantity the quotient is simply V or
velocity. A like division for potential, electrostatic and
electro-magnetic gives (1/V), and so on.
The value of the velocity ratio is very nearly 3E10 (sometimes given as
2.98E10) centimeters per second. This is almost exactly that of light
(2.9992E10 centimeters per second.) This is one of the proofs of Clerk
Maxwell's magnetic theory of light. (See Maxwell's Theory of Light.)
[Transcriber's note: The SI metre was defined in 1983 such that the
speed of light in a vacuum is exactly 299,792,458 metres per second or
about 186,282.397 miles per second.]
Ventilation of Armature.
In a dynamo or motor ventilation of the armature is often provided for
by apertures through it in order to prevent heating. This heating is
caused by Foucault currents. By proper disposition of the interior of
the armature with properly disposed vanes and orifices an action like
that of a fan blower can be produced, which by creating a current of air
cools the machine very efficiently.
Verticity, Poles of.
Points upon the earth's surface where the horizontal component of
magnetic force disappears, leaving only the vertical component active.
The term is derived from the verticity of the dipping needle when over
either of them.
561 STANDARD ELECTRICAL DICTIONARY.
Vibration Period.
In electrical resonance the period of a vibration in an electrical
resonator. The length of this period indicates the quality of the
resonator in responding to electrical oscillations by sympathetic
vibration. For conductors of small resistance the period is thus
calculated. Let T be the period of one-half a full vibration; L the
absolute coefficient of self-induction expressed in centimeters or in
henries X 10-9; C the electrostatic capacity of the terminals, also
expressed in the same unit; v the velocity of light in centimeters per
second. Then we have the formula
T = PI * SquareRoot( L * C ) / v
[Transcriber's note: If the inductance is in henries and the capacitance
in farads, frequency in hertz = 1/(2 * PI * squareRoot( L * C ) )]
Vibration, Sympathetic.
A vibration in a cord or other body susceptible of elastic vibration
produced by the vibrations of exactly the same period in a neighboring
vibrating body. Thus if two tuning forks are tuned to precisely the same
pitch, and are placed near each other, if one is sounded it will start
the other into vibration by sympathy.
In electricity its application is found in electric resonance
experiments. The resonator has a definite period of electric resonance,
and is made to give a spark by the exciter of identical period. This is
by what may be called electric sympathetic vibration, and is exactly
analogous to the action of the tuning forks upon each other.
Vibrator, Electro-magnetic.
The make and break mechanism used on induction coils, or other similar
apparatus in which by alternate attractions by and releases from an
electro-magnet an arm or spring is kept in motion. In most cases the
work is done by a single magnet, whose armature is attracted to the
magnet, when the latter is excited, but against the action of a spring
which tends to pull it away from the magnet. In its motions a make and
break action is produced, to give the requisite alternations of
attraction and release. Two electro-magnets may be connected so as
alternately to be excited and keep an arm carrying a mutual armature in
vibration, or the same result may be attained by a polarized relay. The
make and break is illustrated under Bell, Electric--Coil, Induction--
Anvil.
Villari's Critical Value.
Magnetization induced or residual in a wire is diminished on stretching,
provided that the magnetization corresponds to an inducing force above a
certain critical value, known as above; this being (Sir Wm. Thomson)
about 24 times the terrestrial intensity. Below that critical value
tension increases the magnetization of a magnetized wire. The effects of
transverse expansive stress are opposed to those of longitudinal
stretching. (Daniell.)
Viole's Standard of Illuminating Power.
A standard authorized by the International Congress of 1881. It is the
light given by one square centimeter of platinum, melted, but just at
the point of solidification. It is equal to 20 English standard candles
almost exactly.
It has not been very widely accepted, the tendency among photometrists
being to adhere to the old standards, carcel or candle. It is obvious
that actual use of the Viole would be very inconvenient and would
involve expensive apparatus, difficult to work with.
Synonym--Viole.
562 STANDARD ELECTRICAL DICTIONARY.
Vis Viva.
The kinetic energy of a body in motion; "mechanical energy."
Vitreous Electricity.
Positive electricity; the electricity produced on the surface of glass
by rubbing it with silk and other substances. (See Electrostatic
Series.)
The term "positive electricity" should be allowed to supplant it. It is
the analogue and opposite of resinous electricity.
Vitriol, Blue.
A colloquial or trade name for copper sulphate (Cu SO4).
Vitriol, Green.
A colloquial or trade name for ferrous sulphate (Fe SO4).
Vitriol, White.
A colloquial or trade name for zinc sulphate (Zn SO4).
Volt.
The practical unit of electro-motive force or potential difference. It
may be referred to various data.
An electro-motive force of one volt will cause a current of one ampere
to flow through a resistance of one ohm.
A condenser of one farad capacity charged with one coulomb will have a
rise of potential of one volt.
The cutting of 100,000,000 lines of force per second by a conductor
induces one volt E. M. F.
A Daniell's battery gives an E. M. F. of 1.07 volts; about the most
familiar approximate standard that can be cited.
It is equal to 1/300 absolute electrostatic unit.
It is equal to 1E8 absolute electro-magnetic units.
[Transcriber's note: The SI definition of a volt: The potential
difference across a conductor when a current of one ampere dissipates
one watt of power.]
Voltage.
Potential difference or electro-motive force expressed in volts; as a
voltage of 100 volts. Thus voltage may express the electro-motive force
absorbed in a conductor, while electro-motive force is a term generally
applied where it is produced, evolved or present in the object. The term
voltage of a lamp expresses simply the volts required, but does not
suggest the possession of electromotive force.
563 STANDARD ELECTRICAL DICTIONARY.
Voltage, Terminal.
The voltage or potential difference at the terminals of an electric
current generator, such as a dynamo, as distinguished from the total
electro-motive force of the dynamo or generator.
In batteries the distinction is not generally made in practice; the
total electro-motive force of the battery is made the basis of
calculations.
Voltaic. adj.
This adjective is used to qualify a great many things appertaining to or
connected with current electricity. It is derived from Volta, the
inventor of the voltaic battery, and now tends to displace the term
"galvanic," formerly in general use.
Voltaic Alternatives.
A term used in electro-therapeutics or medical electricity to indicate
an alternating battery current.
Synonym--Alternative current.
Voltaic Effect.
The potential difference developed by contact of different conductors.
It is the basis of the contact theory, q. v., of electricity, although
it may be accepted as the expression for a condition of things by those
who reject the above theory. This potential difference is slight when
the conductors are separated, but it is calculated that it would be
enormous could the metals be so quickly separated as to hold each its
own charge.
Thus if a copper and a zinc plate are assumed to be in contact, really
1/20000000 centimeter or 1/50000000 inch apart, they may be treated as a
pair of condenser plates. Being so near, their density of charge, which
is a strongly bound charge, is enormous. If it were possible to separate
them without permitting any discharge, their potential would rise by the
separation, on the principle of Epinus' condenser, q. v., to such an
extent that they would spark through twenty feet of air. (See Volta's
Fundamental Experiment.)
Voltaic Electricity.
Electricity of low potential difference and large current intensity;
electricity such as produced by a voltaic battery; current or dynamic
electricity as opposed to static electricity.
Voltameter.
In general an apparatus for determining the quantity of electricity
passing through a conductor by measuring the electrolytic action it can
perform.
Voltameter, Copper.
An apparatus which may be of similar construction with the silver
voltameter (see Voltameter, Silver), but in which a copper anode and a
solution of copper sulphate are substituted for the silver anode and
silver nitrate solution. One coulomb corresponds to .329 milligram or
.005084 grain of copper deposited. It is not accepted as of as high a
standard as the silver voltameter.
The electrodes should be placed half an inch from each other. Two square
plate electrodes may conveniently be used, and not less than two square
inches on each plate should be the area per ampere of current.
564 STANDARD ELECTRICAL DICTIONARY.
Voltameter, Differential, Siemens'.
A volume or gas voltameter with duplicate eudiometers and pairs of
electrodes. It is used for determining the resistance of the platinum
conductor used in his pyrometer. A current divides between the two
voltameters; in one branch of the circuit the platinum conductor is
placed, in the other a known resistance. The current strength varying
inversely with the resistance, the resistances of the two conductors are
inversely proportional to the gas evolved.
Voltameter, Gas.
A voltameter whose indications are based on the electrolysis of water,
made an electrolyte by the addition of sulphuric acid. The gases evolved
are measured. It may take several forms.
In one form it is an apparatus consisting of a single eudiometer or
graduated glass tube with upper end closed and its lower end or mouth
open, collecting the mixture of hydrogen and oxygen.
In the form shown in the cut three tubes are connected, the side tubes
representing eudiometers. For each side tube there is a platinum
electrode. In this apparatus the oxygen and hydrogen are connected in
opposite tubes. A is an open tube filled with dilute sulphuric acid. By
opening the cocks on B and C they can both be completely filled with
acid. As shown in the cut, this operation is not yet completed. The
hydrogen alone may in this case be measured.
The mixed gas voltameter has only one eudiometer.
The exact equivalents are only approximately known. The volume of mixed
gases per coulomb is given as .1738 cubic centimeters (Ayrton); .172
cubic centimeters (Hospitalier); and other values by other authorities.
The hydrogen is equal to 1/3 of the mixed gases almost exactly.
Synonyms--Volume Voltameter--Sulphuric Acid Voltameter.
The gas is measured at 0º (32º F.) and 76 centimeters, or 30 inches
barometer.

Fig. 341. GAS VOLTAMETER.
565 STANDARD ELECTRICAL DICTIONARY.
If the gas is measured in cubic inches, the temperature in degrees F.,
and the barometric height in inches, the following formula may be used
for reduction to standard pressure and temperature. It is the volume
corresponding to one coulomb.
( .01058 * 30 * (491 + Fº - 32) ) / (h* 491)
For the metric measurements and degrees C.
(.1738 * 76 * (273 + Cº)) / (h X 273)
Voltameter, Silver.
An apparatus consisting of a platinum vessel containing a solution of
silver nitrate into which solution a silver anode dips, whose end is
wrapped in muslin to prevent the detachment of any particles. When a
current is passed by connecting one terminal to the dish and the other
to the rod, securing a proper direction of current, silver will be
deposited on the dish and the same amount will be dissolved from the
rod. The dish is weighed before and after the test. Its increase in
weight gives the silver deposited.

FIG. 342. SILVER VOLTAMETER.
In the cut Ag is the silver anode, Pt is the platinum dish, r is the
conducting rod, p is a wooden standard, Cu is a copper plate on which
the dish rests and which also serves as a conductor and contact surface,
b is a muslin cloth to place over the silver plate to prevent detached
particles falling in the dish; s s' are the binding screws.
The weight of silver corresponding to a coulomb is given variously by
different authorities. Ayrton and Daniell take 1.11815 milligrams or
.017253 grain of metallic silver. Other determinations are as follows:
1.1183 milligrams (Kohlrausch).
1.124 " (Merscart).
The solution of silver nitrate should be from 15 to 30 per cent. of
strength. The current should not exceed one ampere per six square
inches; or in other words not more than about 3/1000 grain of silver
should be deposited per second on a square inch area of the dish. The
edge of the silver disc or anode should be about equidistant from the
side and bottom of the dish. The latter notes are due to Lord Rayleigh.
566 STANDARD ELECTRICAL DICTIONARY.
Voltameter, Weight.
A voltameter in which the amount of decomposition is determined by
weighing the products, or one of the products of the electrolysis. The
titles Voltameter, Copper, and Voltameter, Silver, may be cited.

Fig. 343. WEIGHT VOLTAMETERS.
In the cuts are shown examples of weight gas voltameters. These are
tubes light enough to be weighed when charged. Each contains a
decomposition cell T, with its platinum electrodes, and charged with
dilute sulphuric acid, while t is calcium chloride or other drying agent
to collect any water carried off as vapor or as spray by the escaping
gases; c are corks placed in position when the weighing is being
executed, so as to prevent the calcium chloride from absorbing moisture
from the air.
In use the tubes are weighed. They are then connected to the circuit,
after removal of the corks, and the decomposition proceeds. After a
sufficient time they are removed, the corks put in place, and they are
weighed again. The loss gives the water decomposed.
The water corresponding to one coulomb is
.09326 milligram .001430 grain, Ayrton,
.092 " Hospitalier,
.0935 " Daniell.
567 STANDARD ELECTRICAL DICTIONARY.
Voltametric Law.
The law on which voltameters are based. The amount of chemical
decomposition produced by an electric current in a given electrolyte is
proportional to the quantity of electricity passed through the solution.

Fig. 344. VOLTA'S FUNDAMENTAL EXPERIMENT.
Volta's Fundamental Experiment.
The moistened finger is placed on the upper plate of a condensing or
electrophorous electroscope. The other hand holds a plate of zinc z,
soldered to a plate of copper c. The lower plate is touched with the
copper. On removing the cover the gold leaves l diverge and with
negative electricity. Hence zinc is supposed to be positively
electrified when in contact with copper. The experiment is used to
demonstrate the contact theory of electricity.
568 STANDARD ELECTRICAL DICTIONARY.
Volta's Law of Galvanic Action.
The electro-motive force between any two metals in an electro-chemical
series (see Electro-Chemical Series) is equal to the sum of the
electro-motive forces between all the intervening metals.
Volta's Law of Thermo-electricity.
In a compound circuit, consisting of a number of different metals, all
points of which are at the same temperature, there is no current.
Volt, B. A.
The volt based on the B. A. ohm. It is equal to .9889 legal volt.
Volt, Congress.
The volt based upon the congress or legal ohm; the legal volt.
Volt-coulomb.
The unit of electric work; the watt-second; it is equivalent to
1.0E7 ergs.
.24068 gram degree C. (calorie)
.737337 foot lbs.,
.00134 horse power seconds.
Volt Indicator.
A form of easily read voltameter for use in electric light stations and
for similar work.
Volt, Legal.
The legal volt based upon the legal ohm. It is equal to 1.00112 B. A.
volt.
Voltmeter.
An instrument for determining the potential difference of any two
points.
In many cases it is a calibrated galvanometer wound with a coil of high
resistance. The object to be attained is that it shall receive only an
insignificant portion of current and that such portion shall suffice to
actuate it. If connected in parallel with any portion of a circuit, it
should not noticeably diminish its resistance.
The divisions into which ammeters range themselves answer for
voltmeters. In practice the same construction is adopted for both. The
different definitions of ammeters in disclosing the general lines of
these instruments are in general applicable to voltmeters, except that
the wire winding of the coils must be of thin wire of great length. The
definitions of ammeters may be consulted with the above understanding
for voltmeters.
In the use made of voltmeters there is a distinction from ammeters. An
ammeter is a current measurer and all the current measured must be
passed through it. But while a voltmeter is in fact a current measurer,
it is so graduated and so used that it gives in its readings the
difference of potential existing between two places on a circuit, and
while measuring the current passing through its own coils, it is by
calibration made to give not the current intensity, but the
electro-motive force producing such current.
In use it may be connected to two terminals of an open circuit, when as
it only permits an inconsiderable current to pass, it indicates the
potential difference existing between such points on open circuit. Or it
may be connected to any two parts of a closed circuit. Owing to its high
resistance, although it is in parallel with the intervening portion of
the circuit, as it is often connected in practice, it is without any
appreciable effect upon the current. It will then indicate the potential
difference existing between the two points.
569 STANDARD ELECTRICAL DICTIONARY.
Voltmeter, Battery.
A voltmeter for use in running batteries. In one form (Wirt's) it is
constructed for a low range of voltage, reading up to two and a half
volts and having exactly one ohm resistance, thus giving the battery
some work to do.
Voltmeter, Cardew.
A voltmeter in which the current passing through its conductor heats
such conductor, causing it to expand. Its expansion is caused to move an
index needle. By calibration the movements of the needle are made to
correspond to the potential differences producing the actuating currents
through it. The magnetic action of the current plays no part in its
operation. It is the invention of Capt. Cardew, R. E.
The construction of the instrument in one of its most recent forms is
shown in the cut. On each side of the drum-like case of the instrument
are the binding screws. These connect with the blocks m and n. To these
the fine wire conductor is connected and is carried down and up over the
two pulleys seen at the lowest extremity, its centre being attached to
c. From c a wire is carried to the drum p, shown on an enlarged scale on
the left of the cut. A second wire from the same drum or pulley connects
to the spring S. The winding of the two wires is shown in the separate
figure of c, where it is seen that they are screwed fast to the
periphery of the little drum, and are virtually continuations of each
other. By the screw A the tension of the spring S is adjusted.
On the shaft of the little drum p is a pinion, which works into the
teeth of the cog-wheel r. The shaft of r is extended through the dial
of the instrument, and carries an index. The dial is marked off for
volts; g g and h h are standards for carrying the pulleys.
570 STANDARD ELECTRICAL DICTIONARY.
The action of the instrument is as follows. The current passing through
the wire heats it. This current by Ohm's law is proportional to the
electro-motive force between the terminals. As it is heated it expands
and as it cools contracts, definite expanding and contracting
corresponding to definite potential differences. As the wire expands and
contracts the block or pin c moves back and forth, thus turning the drum
p and cogwheel r one way or permitting it to turn the other way under
the pull of the spring S.