Page:Elementary Text-book of Physics (Anthony, 1897).djvu/389

§ 312] more consistent results. It is found that $$v,$$ considered as a velocity, is about 3.1010 centimetres in a second. This velocity agrees very closely with the velocity of light.

An experiment was executed by Rowland in which this velocity $$v$$ was obtained by comparison with the actual velocity of a moving charge. The principle of the experiment is as follows: If we consider an indefinitely extended plane surface on which the surface density of electrification is $$\sigma$$ measured in electrostatic units, or $$\frac{\sigma}{v}$$ measured in electromagnetic units, since the ratio of the electrostatic to the electromagnetic unit of quantity is $$v;$$ and conceive it to move in its own plane with a velocity $$x;$$ the charge moving with it may be considered as the equivalent of a current in that surface, the strength of which, measured by the quantity of electricity which crosses a line of unit length, perpendicular to the direction of movement, in unit time, is $$\frac{\sigma x}{v}\cdot$$ The force due to such a current on a magnet may be calculated. Conversely, if the force on the magnet be observed, and the surface density $$\sigma$$ and the velocity $$x$$ be also measured, the value of $$v$$ may be calculated. The probability of such an action as the one here described was stated by Maxwell.

The experiment by which Rowland verified Maxwell's view consisted in rotating a disk cut into numerous sectors, each of which was electrified, under an astatic magnetic needle. During the rotation of the disk, a deflection of the needle was observed, in the same sense as that in which it would have moved if a current had been flowing about the disk in the direction of its rotation. From the measured values of the deflecting force, of the surface density of electrification on the disk, and the velocity of rotation, Rowland calculated a value of $$v$$ which lies between those given by Weber and Maxwell.

312. Oscillatory Discharge of a Condenser.—If a condenser be discharged through a circuit, the current in the circuit will manifestly depend on the original difference of potential between the