When the coils are all on, the hoop should be attached to the uprights with small brass screws driven through holes in the hoop and into the wood. The wire is bound to the hoop by means of threads or silk passed through each pair of holes in the hoop, and then tied fast. Fine insulated wire may be used in place of the thread, but care should be taken that the insulation is in perfect shape on both the binding and coil wires; otherwise a short-circuit will quickly destroy the value of the coils.
The hoop should not touch the base-block, but should clear it by a quarter or half an inch. Make the coil ends fast (as described for the astatic galvanometer and illustrated at Fig. 9) by means of binding-posts. The wires need not be carried over the top of the block, but may run through holes under the hoop and along grooves cut in the under side of the block and leading to the foot of the binding-posts.
The graduated card should be made from a piece of stout bristol-board or heavy card-board having a smooth, hard surface. It is laid out with a pencil or pen compass, as shown at Fig. 12, and should be three inches in diameter. The card is placed on the wood strip or ledge, so that the zero marks will be at the front and rear, or at right angles to the hoop and coils of wire. The compass needle, when at rest, should lie parallel with the coils, so that the current will deflect the needle and send the indicator around to one side or the other of zero, according to the direction in which the current is passing through the coils.
This is more clearly shown at Fig. 13. The circle represents the outside diameter of the card; the dark cross-piece, the magnetic needle; and the pointed indicator, a stiff paper, or very thin brass or copper strip, cut and attached to the needle with shellac or paraffine.
When at rest the magnetic needle should be parallel to the coils. To insure this the instrument must be moved so that the lines of wire forming the coil will run North and South. Otherwise the N-seeking end of the magnetic shaft will point to North, irrespective of the position occupied by the wire coil.
The magnetic needle may be made as described for the compass (see chapter iv., Magnets and Induction Coils). It should be arranged to rest on a brass pivot pressed down into the cross-piece of wood.