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

by T. O'Conor Sloane

Fig. 270. ILLUSTRATION OF THE FALL AND
REDEVELOPMENT OF POTENTIAL IN AN ELECTRIC CIRCUIT.
The fall of potential in a circuit in portions of it is proportional to
the resistance of the portions in question. This is shown in the
diagram. The narrow lines indicate high and the broad lines low
resistance. The fall in different portions is shown as proportional to
the resistance of each portion.

Fig. 271. DIAGRAM OF FALL OF POTENTIAL IN A
CONDUCTOR OF UNEVEN RESISTANCE.
Potential, Magnetic.
The magnetic potential at any point of a magnetic field expresses the
work which would be done by the magnetic forces of the field on a
positive unit of magnetism as it moves from that point to an infinite
distance therefrom. The converse applies to a negative unit.
It is the exact analogue of absolute electric potential.
The potential at any point due to a positive pole m at a distance r is
m/r;. that due to a negative pole - m at a distance r' is equal to
-m/r';. that due to both is equal to m/r - m/r' or m(1/r - 1/r').
Like electric potential and potential in general, magnetic potential
while numerically expressing work or energy is neither, although often
defined as such.
432 STANDARD ELECTRICAL DICTIONARY.
Potential, Negative.
The reverse of positive potential. (See Potential, Positive.)
Potential, Positive.
In general the higher potential. Taking the assumed direction of lines
of force, they are assumed to be directed or to move from regions of
positive to regions of negative potential. The copper or carbon plate of
a voltaic battery is at positive potential compared to the zinc plate.
Potential, Unit of Electric.
The arbitrary or conventional potential--or briefly, the potential of a
point in an electric field of force--is, numerically, the number of ergs
of work necessary to bring a unit of electricity up to the point in
question from a region of nominal zero potential--i. e., from the
surface of the earth. (Daniell.) This would give the erg as the unit of
potential.
Potential, Zero.
The potential of the earth is arbitrarily taken as the zero of electric
potential.
The theoretical zero is the potential of a point infinitely distant from
all electrified bodies.

Fig. 272. DIAGRAM OF POTENTIOMETER CONNECTIONS.
Potentiometer.
An arrangement somewhat similar to the Wheatstone Bridge for determining
potential difference, or the electro-motive force of a battery. In
general principle connection is made so that the cell under trial would
send a current in one direction through the galvanometer. Another
battery is connected, and in shunt with its circuit the battery under
trial and its galvanometer are connected, but so that its current is in
opposition. By a graduated wire, like that of a meter bridge, the
potential of the main battery shunt can be varied until no current
passes. This gives the outline of the method only.
433 STANDARD ELECTRICAL DICTIONARY.
In the cut A B is the graduated potentiometer wire through which a
current is passed in the direction of the arrow. E is the battery under
trial, placed in opposition to the other current, with a galvanometer
next it. Under the conditions shown, if the galvanometer showed no
deflection, the E. M. F. of the battery would be to the E. M. F. between
the ends of the potentiometer wire, 1 . . . . .10, as 1.5 the distance
between the points of connection, A and D of the battery circuit, is to
10, the full length of the potentiometer wire.
Poundal.
The British unit of force; the force which acting on a mass of one pound
for one second produces an acceleration of one foot.
[Transcriber's note: The force which acting on a mass of one pound
produces an acceleration of ONE FOOT PER SECOND PER SECOND.]
Power. Activity;
the rate of activity, of doing work, or of expending energy. The
practical unit of electric power is the volt-ampere or watt, equal to
1E7 ergs per second. The kilowatt, one thousand watts or volt-amperes,
is a frequently adopted unit.
Power, Electric.
As energy is the capacity for doing work, electric energy is represented
by electricity in motion against a resistance. This possesses a species
of inertia, which gives it a species of kinetic energy. To produce such
motion, electro- motive force is required. The product of E. M. F. by
quantity is therefore electric energy. (See Energy, Electric.)
Generally the rate of energy or power is used. Its dimensions are
( ( (M^.5)*(L^.5) ) / T ) * ( ( (M^.5) *(L^1.5) )/( T^2) )
(intensity or current rate) * (electro-motive force or potential)
= (M * (L^2) ) / (T^3),
which are the dimensions of rate of work or activity. The practical unit
of electric rate of energy or activity is the volt-ampere or watt. By
Ohm's law, q. v., we have C = E/R (C = current; E = potential difference
or electro-motive force; R = resistance.) The watt by definition = C*E.
By substitution from Ohm's formula we deduce for it the following
values: ((C^2) * R) and ((E^2) /R). From these three expressions the
relations of electric energy to E.M.F., Resistance, and Current can be
deduced.
Power of Periodic Current.
The rate of energy in a circuit carrying a periodic current. In such a
circuit the electro-motive force travels in advance of the current it
produces on the circuit. Consequently at phases or intervals where,
owing to the alternations of the current, the current is at zero, the
electro-motive force may be quite high. At any time the energy rate is
the product of the electro-motive force by the amperage. To obtain the
power or average rate of energy, the product of the maximum
electro-motive force and maximum current must be divided by two and
multiplied by the cosine of the angle of lag, which is the angle
expressing the difference of phase.
[Transcriber's note; The voltage phase will lead if the load is
inductive. The current phase will lead if the load is capacitive.
Capacitors or inductors may be introduced into power lines to correct
the phase offset introduced by customer loads.]
434 STANDARD ELECTRICAL DICTIONARY.
Pressel.
A press-button often contained in a pear-shaped handle, arranged for
attachment to the end of a flexible conductor, so as to hang thereby. By
pressing the button a bell may be rung, or a distant lamp may be
lighted.
Pressure.
Force or stress exerted directly against any surface. Its dimensions are
force/area or ((M*L)/(T^2)) / (L^2) = M/(L* (T^2)).
Pressure, Electric.
Electro-motive force or potential difference; voltage. An expression of
metaphorical nature, as the term is not accurate.
Pressure, Electrification by.
A crystal of Iceland spar (calcium carbonate) pressed between the
fingers becomes positively electrified and remains so for some time.
Other minerals act in a similar way. Dissimilar substances pressed
together and suddenly separated carry off opposite charges. This is
really contact action, not pressure action.
Primary.
A term used to designate the inducing coil in an induction coil or
transformer; it is probably an abbreviation for primary coil.
Primary Battery.
A voltaic cell or battery generating electric energy by direct
consumption of material, and not regenerated by an electrolytic process.
The ordinary voltaic cell or galvanic battery is a primary battery.
Prime. vb.
To impart the first charge to one of the armatures of a Holtz or other
influence machine.

Fig. 273. PRIME CONDUCTOR AND PROOF PLANE.
435 STANDARD ELECTRICAL DICTIONARY.
Prime Conductor.
A metal or metal coated sphere or cylinder or other solid with rounded
ends mounted on insulating supports and used to collect electricity as
generated by a frictional electric machine.
According to whether the prime conductor or the cushions are grounded
positive or negative electricity is taken from the ungrounded part.
Generally the cushions are grounded, and the prime conductor yields
positive electricity.
Probe, Electric.
A surgeon's probe, designed to indicate by the closing of an electric
circuit the presence of a bullet or metallic body in the body of a
patient.
Two insulated wires are carried to the end where their ends are exposed,
still insulated from each other. In probing a wound for a bullet if the
two ends touch it the circuit is closed and a bell rings. If a bone is
touched no such effect is produced. The wires are in circuit with an
electric bell and battery.
Projecting Power of a Magnet.
The power of projecting its lines of force straight out from the poles.
This is really a matter of magnetic power, rather than of shape of the
magnet. In electromagnets the custom was followed by making them long to
get this effect. Such length was really useful in the regard of getting
room for a sufficient number of ampere turns.
436 STANDARD ELECTRICAL DICTIONARY.

Fig. 274. PRONY BRAKE.
Prony Brake.
A device for measuring the power applied to a rotating shaft. It
consists of a clamping device to be applied more or less rigidly to the
shaft or to a pulley upon it. To the clamp is attached a lever carrying
a weight. The cut shows a simple arrangement, the shaft A carries a
pulley B to which the clamp B1 B2 is applied. The nuts C1 C2 are used
for adjustment.
A weight is placed in the pan E attached to the end of the lever D. The
weight and clamp are so adjusted that the lever shall stand horizontally
as shown by the index E. If we call r the radius of the pulley and F the
friction between its surface and the clamp, it is evident that r F, the
moment of resistance to the motion of the pulley, is equal to the weight
multiplied by its lever arm or to W*R, where W indicates the weight and
R the distance of its point of application from the centre of the pulley
or r*F = R*W. The work represented by this friction is equal to the
distance traveled by the surface of the wheel multiplied by the
frictional resistance, or is 2*PI*r*n*F, in which n is the number of
turns per minute. But this is equal to 2*PI*R*W. These data being known,
the power is directly calculated therefrom in terms of weight and feet
per minute.
Proof-plane.
A small conductor, usually disc shaped, carried at the end of an
insulating handle. It is used to collect electricity by contact, from
objects electrostatically charged. The charge it has received is then
measured (see Torsion Balance) or otherwise tested. (See Prime
Conductor.)
Proof-sphere.
A small sphere, coated with gold-leaf or other conductor, and mounted on
an insulated handle. It is used instead of a proof-plane, for testing
bodies whose curvature is small.