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

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


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 be the disk, the guard-ring with the rest of the conducting vessel attached to it, and the large disk of one of these accumulators, and let , and 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 or , and to suppose to be the inner and the outer conducting surface. in this case being understood to surround .

Let the following connexions be made.

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

(1) Let be connected with and and with , the electrode of a Leyden jar, and let be connected with and and with the earth.

(2) Let and be insulated from .

(3) Let be insulated from and , and from and .

(4) Let and be connected with and and with the earth.

(5) Let be connected with .