Page:A Treatise on Electricity and Magnetism - Volume 2.djvu/403

 

capacity of the jar into the difference of potential of its coatings. The capacity of the jar was determined by comparison with that of a sphere suspended in an open space at a distance from other bodies. The capacity of such a sphere is expressed in electrostatic measure by its radius. Thus the capacity of the jar may be found and expressed as a certain length. See Art. 227.

The difference of the potentials of the coatings of the jar was measured by connecting the coatings with the electrodes of an electrometer, the constants of which were carefully determined, so that the difference of the potentials, $E$, became known in electrostatic measure.

By multiplying this by $c$, the capacity of the jar, the charge of the jar was expressed in electrostatic measure.

To determine the value of the charge in electromagnetic measure, the jar was discharged through the coil of a galvanometer. The effect of the transient current on the magnet of the galvanometer communicated to the magnet a certain angular velocity. The magnet then swung round to a certain deviation, at which its velocity was entirely destroyed by the opposing action of the earth's magnetism.

By observing the extreme deviation of the magnet the quantity of electricity in the current may be determined in electromagnetic measure, as in Art. 748, by the formula

$Q={\frac {H}{G}}{\frac {T}{\pi }}2\sin {\frac {1}{2}}\theta$,

where $Q$ is the quantity of electricity in electromagnetic measure. We have therefore to determine the following quantities:—

$H$, the intensity of the horizontal component of terrestrial magnetism; see Art. 456.

$G$, the principal constant of the galvanometer; see Art. 700.

$T$, the time of a single vibration of the magnet; and

$\theta$, the deviation due to the transient current.

The value of $\nu$ obtained by MM. Weber and Kohlrausch was

$\nu =310740000$ metres per second.

The property of solid dielectrics, to which the name of Electric Absorption has been given, renders it difficult to estimate correctly the capacity of a Leyden jar. The apparent capacity varies according to the time which elapses between the charging or discharging of the jar and the measurement of the potential, and the longer the time the greater is the value obtained for the capacity of the jar.