Prof. J. Perry lately delivered a lecture on this subject at the Society of Arts, London, which contains in an epitomized form the salient points of the hopes and fears of the more sanguine spirits of the electrical world. Prof. Perry is one of the two professors who have been dubbed the "Japanese Twins," and whose insatiate love of work induced one of our most celebrated men of science to say that they caused the center of experimental research to tend toward Tokyo instead of London. Professors Ayrton and Perry have for some time been again resident in England, but it is evident that they did not leave any of their energy in Japan, for those who know them intimately, know that they are pursuing numerous original investigations, and that so soon as one is finished, another is commenced. It would have been difficult then to have found an abler exponent of the future of electricity.

Prof. Perry, after referring to what might have been said of the great things physical science has done for humanity, plunged into his subject. The work to be done was vast, and the workers altogether out of proportion to the task.

The methods of measurement of electricity are not generally understood. Perhaps when electricity is supplied to every house in the city at a certain price per horse power, and is used by private individuals for many different purposes, this ignorance will disappear. Electrical energy is obtained in various ways, but the generators get heated; and one great object of inventors is to obtain from machines as much as possible electrical energy of the energy in the first place supplied to such machine. The lecturer called particular attention to the difference between electricity and electrical energy, and attempted to drive home the fundamental conceptions of electrical science by the analogies derivable from hydraulics. A miller speaks not only of quantity of water, but also of head of water. The statement then of quantity of electricity is insufficient, except we know the electrical property analogous to head of water, and which is termed electrical potential. A small quantity of electricity of high potential is similar to a small quantity of water at high level. The analogies between water and electricity were collected in the form of a table shown on a wall sheet as follows:

 We Want to Use Water. We Want to Use Electricity. 
1. Steam pump burns coal, 1. Generator burns zinc, or and lifts water to a higher uses mechanical power, and level. lifts electricity to a higher level or potential.
2. Energy available is 2. Energy available is amount of water lifted x amount of electricity x difference difference of level. of potential.
3. If we let all the water 3. If we let all the electricity flow away through channel flow through a wire from one to lower level without doing screw of our generator to the work, its energy is all other without doing work, all converted into heat because the electrical energy is of frictional resistance of converted into heat because of pipe or channel. resistance of wire.
4. If we let water work a 4. If we let our electricity hoist as well as flow through work a machine as well as channels, less water flows flow through wires, less flows than before, less power is than before, less power is wasted in friction. wasted through the resistance of the wire.
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:

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.