the E.M.F. set up by a single Daniell's cell; the exact amount being 1·079 volt, or 1-1/12 volt very nearly. The E.M.F. of the Leclanché is very nearly 1·6 volt, or nearly 1 volt and 2/3. Thus in [Fig. 15], which illustrates 3 Leclanché cells set up in series, we should get
| 1·6 | volt |
| 1·6 | " |
| 1·6 | " |
| 4·8 | volts |
as the total electromotive force of the combination.
Fig. 15.
§ 39. The current, or amplitude of the continuous vibrations kept up in the circuit, depends upon two things: 1st, the electromotive force; 2nd, the resistance in the circuit. There is a certain amount of resemblance between the flow of water under pressure and electricity in this respect. Let us suppose we have a
constant "head" of water at our disposal, and allow it to flow through a tube presenting 1 inch aperture. We get a certain definite flow of water, let us say 100 gallons of water per hour. More we do not get, owing to the resistance opposed by the narrowness of the tube to a greater flow. If now we double the capacity of the exit tube, leaving the pressure or "head" of water the same, we shall double the flow of water. Or we may arrive at the same result by doubling the "head" or pressure of water, which will then cause a double quantity of water to flow out against the same resistance in the tube, or conductor. Just in the same way, if we have a given pressure of electric strain, or E.M.F., we can get a greater or lesser flow or "current" by having less or more resistance in the circuit. The standard of flowing current is called an Ampère; and 1 ampère is that current which, in passing through a solution of sulphate of copper, will deposit 18·35 grains of copper per hour. The unit of resistance is known as an Ohm. The resistance known as 1 ohm is very nearly that of a column of mercury 1 square millimètre (1/25 of an inch) in section, and 41¼ inches in height; or 1 foot of No. 41 gauge pure copper wire, 33/10000 of an inch in diameter, at a temperature of 32° Fahr., or 0° Centigrade.
§ 40. Professor Ohm, who made a special study of the relative effects of the resistance inserted in the circuit, the electromotive force, and the current produced, enunciated the following law, which, after him, has been called "Ohm's Law." It is that if we divide the number of electromotive force units (volts) employed by
the number of resistance units (ohms) in the entire circuit, we get the number of current units (ampères) flowing through the circuit. This, expressed as an equation is shown below: