A.—Coil of wire.
B.—Magnetized needle.
C. Pointer.
Fig. 2.
All these parts can now be fixed together. In the middle of the broad side of the coil of wire you must push aside the strands in such a manner that holes can be made through the cardboard hoop. Pass the two ends of the thick wire, to which the needle is attached through these holes, so that the needle is then suspended within the coil. Take care to make these holes large enough to allow the axles of the needle to revolve freely, as the whole success of your apparatus depends upon the needle turning easily within the coil, which should now appear as in [Fig. 3].
Fasten the coil upon the back of the dial, one end of the protruding axle passing through the hole you have already made in the wood. Then glue the upright firmly in its groove, with the other end of the axle through its corresponding hole. Take great care that the needle can revolve freely within the coil. Fasten the pointer to the end of the axle passing through the dial, and in this latter drive two pins or wooden pegs (P P) on either side of the pointer, in order that it may only move within a limited arc (B, [Fig. 1]).
Fig. 3.—Revolving needle in coil.