Page:Elementary Text-book of Physics (Anthony, 1897).djvu/331

§ 274] If heat and difference of potential be measured in absolute units, this equation enables us to determine the absolute unit of current. The system of units here used is the electrostatic system. The dimensions of current strength in the electrostatic system are obtained from this equation. We have $$[I] = \left[ \frac{Q}{t} \right] = M^{\frac{1}{2}} L^{\frac{3}{2}} T^{-2}.$$

273. Electromotive Force.—We may consider the current as an operation by which energy is transformed in the conductor, either by the transfer of electricity through it or by the entrance of tubes of force into it and their disappearance within it. In the example before us, we have assumed that the transfer or the movement of the tubes was due to some cause which set up a difference of potential between parts of the conductor. This condition is not necessary for the maintenance of a current; in certain circumstances energy may be expended in a conductor, without the existence of a finite difference of potential between neighboring parts of the conductor. The power of establishing and sustaining the conditions which make a continuous expenditure of energy possible is called electromotive force.

The energy expended in unit time in a circuit carrying the current $$I,$$ and in which the electromotive force is $$E,$$ is In a circuit containing a voltaic battery, if two points on the circuit outside the battery be tested by an electrometer, a difference of potential between them will be found. If the circuit be broken between the two points considered, the difference of potential between them becomes greater. This maximum difference of potential is the sum of finite differences of potential supposed to be due to molecular interactions at the surfaces of contact of different substances in the circuit, and is the measure of the electromotive force of the battery.

274. Ohm's Law.—In § 252 it was remarked that a body is distinguished as a good or a poor conductor by the rate at which it