The difference between potential and electro-motive force was explained thus: "difference of potential" is analogous with "difference of pressure" or "head" of water, howsoever produced; whereas electromotive force is analogous with the difference of pressure before and behind a slowly moving piston of the pump employed by an unfortunate miller to produce his water supply. Electricians have very definite ideas upon the subject they are working at, and especial attention is paid to the measurements on which their work depends. Examples of these measurements were shown by the following tables on wall sheets:
ELECTRICAL MAGNITUDES (SOME RATHER APPROXIMATE).
Resistance of
One yard of copper wire, one-eighth
of an inch diameter...............................0.002 ohms.
One mile ordinary iron telegraph wire, .........10 to 20 "
Some of our selenium cells ............. 40 to 1,000,000 "
A good telegraph insulator ........... 4,000,000,000,000 "
Electro-motive force of
A pair of copper-iron junctions at a
difference of temperature of 1 deg. Fah......... =0.0000 volt.
Contact of zinc and copper ..................... =0.75 "
One Daniell's cell ............................. =1.1 "
Mr. Latimer Clark's standard cell .............. =1.45 "
One of Dr. De la Hue's batteries ...... =11,000 "
Lightning flashes probably many millions of volts.
Current measured by us in some experiments:
Using electrometer....... = almost infinitely small
currents.
Using delicate galvanometer =0.00,000,000,040 weber.
Current received from Atlantic
cable, when 25 words per minute
are being sent ................ = 0.000,001 weber
Current in ordinary land telegraph
lines ......................... = 0.003 weber
Current from dynamo machine.... = 5 to 100 weber
In any circuit, current in webers = electro-motive force in volts / resistance in ohms.
RATE OF PRODUCTION OF HEAT, CALCULATED IN THE SHAPE OF HORSE-POWER.
In the whole of a circuit=current in webers x electro-motive force in volts / 746. In any part of circuit=current in webers x difference of potential at the two ends of the part of the circuit in question / 746. Or, =square of current in webers x resistance of the part in ohms / 746.
If there are a number of generators of electricity in a circuit, whose electromotive forces in volts are E1, E2, etc., and if there are also opposing electro-motive forces. F1, F2, etc., volts, and if C is the current in webers, R the whole resistance of the current in ohms, P the total horse-power taken at the generators, Q the total horse-power converted into some other form of energy, and given out at the places where there are opposing electro-motive forces, H the total horse-power wasted in heat, because of resistance, then:
