I have employed the following simple apparatus to
illustrate the action of the dynamo and its commutator,
which requires very little mechanical skill to construct,
A disk of wood about a foot in diameter (Fig. 11a) has
a pointed stick driven through its centre, thus mak-
ing a large Japanese top. The disk being very near
one end of the rod which passes through its centre,
such a top will continue to revolve in the same spot on



THE DYNAMO MACHINE.
101



a smooth floor for a long time by merely twirling the
rod between the fingers. Stick now a little flat spool
of insulated copper wires (S 7 ) upon the wooden disk,
lead its wires up the rod, and bind over their bare ends
tightly two little shieldlike pieces of bright sheet cop-
per or brass, so that these shields partly embrace the
rod, being insulated from each other by the wood.
Pig. 11a.
These shields form the ends, B, D, of the moving spool,
S'. On a piece of wood in which a hole is bored to
admit of the top turning run the wires A, C, to the
coil M of the galvanometer, and remove their covering,
so that the bare wire shall touch the ends B and D.
