This apparatus consists of two cylinders, one, A, made of brass, the
other, B, of wood, with a spiral groove. At its end is a copper ring a.
A fine brass wire has one end attached to this ring. Its other end is
fastened at e, and it is wound as shown; n and o are binding screws
connected, one with the cylinder-ring a, the other with the brass
cylinder, A. The current entering at o, traverses the wire on B, as
there the windings are insulated by the grooves, thence it passes to m
and by A, whose metal short circuits all the wire on it, to the
binding-post n. The handle, d, is turned one way or the other to
regulate the length of the wire through which the current must pass. On
each cylinder there is a square head, one of which is shown at c, so
that the handle can be shifted from one to the other as required; to A
if the wire is to be wound on that cylinder, to B if the reverse is
desired.