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: