Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/329

be altered and made equal to that of the same accumulator when $A$ and $C$ are nearer together. If the accumulator with the dielectric plate, and with $A$ and $C$ at distance $x$, is of the same capacity as the same accumulator without the dielectric, and with $A$ and $C$ at distance $x'$, then, if $a$ is the thickness of the plate, and $K$ its specific dielectric inductive capacity referred to air as a standard,

$K={\frac {a}{a+x'-x}}$

The combination of three cylinders, described in Art. 127, has been employed by Sir W. Thomson as an accumulator whose capacity may be increased or diminished by measurable quantities.

The experiments of MM. Gibson and Barclay with this apparatus are described in the Proceedings of the Royal Society, Feb. 2, 1871, and Phil. Trans., 1871, p. 573. They found the specific inductive capacity of paraffin to be 1.975, that of air being unity.