168. Let two conductors A and B be at different potentials, A being at the higher potential and having a charge of positive electricity, while B is at a lower potential and has a charge of negative electricity; then if A is connected to B by a metallic wire the potential of A will begin to diminish and A will lose some of its positive charge, the potential of B will increase and B will lose some of its negative charge, so that in a short time the potentials of A and B will be equalized. During the time in which the potentials of A and B are changing the following phenomena will occur: the wire connecting A and B will be heated and a magnetic field will be produced which is most intense near the wire. If A and B are merely charged conductors, their potentials are equalized so rapidly, and the thermal and magnetic effects are in consequence so transient, that it is somewhat difficult to observe them. If, however, we maintain A and B at constant potentials by connecting them with the terminals of a voltaic battery the thermal and magnetic effects will persist as long as the connection with the battery is maintained, and are then easily observed.