, connected by a wire AMB . Small conducting masses in motion first come in contact with the conductor B , take from it an electric charge, leave contact with B and move along the path BNA , and, transporting with them their charge, come into contact with A and give to it their charge, which returns then to B along the wire AMB .
Now there we have in a sense a closed circuit, since the electricity describes the closed circuit BNAMB; but the two parts of this current are very different. In the wire AMB, the electricity is displaced through a fixed conductor, like a voltaic current, overcoming an ohmic resistance and developing heat; we say that it is displaced by conduction. In the part BNA, the electricity is carried by a moving conductor; it is said to be displaced by convection.
If then the current of convection is considered as altogether analogous to the current of conduction, the circuit BNAMB is closed; if, on the contrary, the convection current is not 'a true current' and, for example, does not act on the magnet, there remains only the conduction current AMB, which is open.
For example, if we connect by a wire the two poles of a Holtz machine, the charged rotating disc transfers the electricity by convection from one pole to the other, and it returns to the first pole by conduction through the wire.
But currents of this sort are very difficult to produce with appreciable intensity. With the means at Ampère's disposal, we may say that this was impossible.