Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/331

as a current of positive electricity from $A$ to $B,$ or a current of negative electricity from $B$ to $A,$ or as a combination of these two currents.

According to Fechner's and Weber's theory it is a combination of a current of positive electricity with an exactly equal current of negative electricity in the opposite direction through the same substance. It is necessary to remember this exceedingly artificial hypothesis regarding the constitution of the current in order to understand the statement of some of Weber's most valuable experimental results.

If, as in Art. 36, we suppose $P$ units of positive electricity transferred from $A$ to $B,$ and $N$ units of negative electricity transferred from $B$ to $A$ in unit of time, then, according to Weber's theory, $P=N$, and $P$ or $N$ is to be taken as the numerical measure of the current.

We, on the contrary, make no assumption as to the relation between $P$ and $N,$ but attend only to the result of the current, namely, the transference of $P+N$ of positive electrification from $A$ to $B$, and we shall consider $P+N$ the true measure of the current. The current, therefore, which Weber would call 1 we shall call 2.

On Steady Currents.

232.] In the case of the current between two insulated conductors at different potentials the operation is soon brought to an end by the equalization of the potentials of the two bodies, and the current is therefore essentially a Transient current.

But there are methods by which the difference of potentials of the conductors may be maintained constant, in which case the current will continue to flow with uniform strength as a Steady Current.

The Voltaic Battery.

The most convenient method of producing a steady current is by means of the Voltaic Battery.

For the sake of distinctness we shall describe Daniell's Constant Battery:—

A solution of sulphate of zinc is placed in a cell of porous earthenware, and this cell is placed in a vessel containing a saturated solution of sulphate of copper. A piece of zinc is dipped into the sulphate of zinc, and a piece of copper is dipped into the sulphate of copper. Wires are soldered to the zinc and to the copper above