45) that they magnetize the part of the iron band passing through the coil.

It had long been known that the discharges from a Leyden jar could magnetize or demagnetize steel needles.

p. 203) noticed that a single electric spark about an inch long thrown on to a circuit of wire in an upper room could magnetize steel needles included in a parallel circuit of wire placed in a cellar 30 ft.

Phys., 1820, 1 5, p. 94) and Davy (Annals of Philosophy, 1821) discovered independently the power of the electric current to magnetize 1 " Memoire sur la theorie mathematique des phenomenes electrodynamiques," Memoires de l'institut, 1820, 6; see also Ann.

The most interesting of his discoveries, now generally known as the " Wiedemann effect," is the following: If we magnetize longitudinally a straight wire which is fixed at one end and free at the other, and then pass an electric current through the wire (or first pass the current and then magnetize), the free end of the wire will twist in a certain direction depending upon circumstances: if the wire is of iron, and is magnetized (with a moderate force) so that its free end has north polarity, while the current through it passes from the fixed to the free end, then the free end as seen from the fixed end will twist in the direction of the hands of a watch; if either the magnetization or the current is reversed, the direction of the twist will be reversed.