5. However long and narrow 5. However long and thin may be the channels, the wires may be, electricity water maybe brought from may be brought from any distance distance, however great, however great, to give to give out almost all its out almost all its original original energy to a hoist. energy to a machine. This requires This requires a great head a great difference of and small quantity of water. potentials and a small current. 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.