Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/332

the surface of the liquid. This combination is called a cell or element of Daniell's battery. See Art. 272.

233.] If the cell is insulated by being placed on a non-conducting stand, and if the wire connected with the copper is put in contact with an insulated conductor $A$, and the wire connected with the zinc is put in contact with $B$, another insulated conductor of the same metal as $A$, then it may be shewn by means of a delicate electrometer that the potential of $A$ exceeds that of $B$ by a certain quantity. This difference of potentials is called the Electromotive Force of the Daniell's Cell.

If $A$ and $B$ are now disconnected from the cell and put in communication by means of a wire, a transient current passes through the wire from $A$ to $B$, and the potentials of $A$ and $B$ become equal. $A$ and $B$ may then be charged again by the cell, and the process repeated as long as the cell will work. But if $A$ and $B$ be connected by means of the wire $C$, and at the same time connected with the battery as before, then the cell will maintain a constant current through $C$, and also a constant difference of potentials between $A$ and $B$. This difference will not, as we shall see, be equal to the whole electromotive force of the cell, for part of this force is spent in maintaining the current through the cell itself.

A number of cells placed in series so that the zinc of the first cell is connected by metal with the copper of the second, and so on, is called a Voltaic Battery. The electromotive force of such a battery is the sum of the electromotive forces of the cells of which it is composed. If the battery is insulated it may be charged with electricity as a whole, but the potential of the copper end will always exceed that of the zinc end by the electromotive force of the battery, whatever the absolute value of either of these potentials may be. The cells of the battery may be of very various construction, containing different chemical substances and different metals, provided they are such that chemical action does not go on when no current passes.

234.] Let us now consider a voltaic battery with its ends insulated from each other. The copper end will be positively or vitreously electrified, and the zinc end will be negatively or resinously electrified.

Let the two ends of the battery be now connected by means of a wire. An electric current will commence, and will in a very short time attain a constant value. It is then said to be a Steady Current.