Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/327

 Now let the vessel $$Bb$$ be put in connexion with the earth. The charge on the disk $$A$$ will no longer be uniformly distributed, but it will remain the same in quantity, and if we now discharge $$A$$ we shall obtain a quantity of electricity, the value of which we know in terms of $$V$$, the original difference of potentials and the measurable quantities $$R, R'$$ and $$A$$.

On the Comparison of the Capacity of Accumulators.

229.] The form of accumulator which is best fitted to have its capacity determined in absolute measure from the form and dimensions of its parts is not generally the most suitable for electrical experiments. It is desirable that the measures of capacity in actual use should be accumulators having only two conducting surfaces, one of which is as nearly as possible surrounded by the other. The guard-ring accumulator, on the other hand, has three independent conducting portions which must be charged and discharged in a certain order. Hence it is desirable to be able to compare the capacities of two accumulators by an electrical process, so as to test accumulators which may afterwards serve as secondary standards.

I shall first shew how to test the equality of the capacity of two guard-ring accumulators.

Let $$A$$ be the disk, $$B$$ the guard-ring with the rest of the conducting vessel attached to it, and $$C$$ the large disk of one of these accumulators, and let $$A', B'$$, and $$C'$$ be the corresponding parts of the other.

If either of these accumulators is of the more simple kind, having only two conductors, we have only to suppress $$B$$ or $$B'$$, and to suppose $$A$$ to be the inner and $$C$$ the outer conducting surface. $$C$$ in this case being understood to surround $$A$$.

Let the following connexions be made.

Let $$B$$ be kept always connected with $$C'$$, and $$B'$$ with $$C$$, that is, let each guard-ring be connected with the large disk of the other condenser.

(1) Let $$A$$ be connected with $$B$$ and $$C'$$ and with $$J$$, the electrode of a Leyden jar, and let $$A'$$ be connected with $$B'$$ and $$C$$ and with the earth.

(2) Let $$A, B,$$ and $$C'$$ be insulated from $$J$$.

(3) Let $$A$$ be insulated from $$B$$ and $$C'$$, and $$A'$$ from $$B'$$ and $$C'$$.

(4) Let $$B$$ and $$C'$$ be connected with $$B'$$ and $$C$$ and with the earth.

(5) Let $$A$$ be connected with $$A'$$.