Fig. 91.
The Spools, F, are glued to C. If you have brass screws, these may be used instead of the spools; they should be left sticking out from C about 1 in. Around the spools or screws, fasten a pasteboard band, G, on which to wind the wire. G may be about 1 in. wide; it should be kept in the circular form by sewing the ends together where they lap. (Read directions in [App. 116].)
191. The Coils on this model are 4 in number. ([See App. 116] for the method of winding.) The first coil is made of coarse wire, No. 18, its ends being joined to the binding-posts, V and W. The second coil has 5 turns of No. 24 insulated copper wire, its ends being joined to W and X. The third coil has 10 turns of the same size wire, No. 24, and is joined to X and Y. The fourth coil has 20 turns of the same joined to Y and Z. If you want to use the galvanometer for quite weak currents, it would be well to make a fifth coil of 20 turns of No. 30 wire, and join it with Z and a new binding-post. The ends of the coils are run through small screw-eyes before passing to X, Y, etc. This is not necessary, it merely keeps them in place.
The Binding-Posts are like [App. 43]. Any other desired style may be used, those of [App. 46] being preferred.
The Hole, H, is 5 in. in diameter. It should be cut out about ½ in. below the center of the circles to allow for D, and for the pin-point which supports the magnetic needle, the poles of which should be in the line passing through the center of the coils. The method of cutting the hole, H, through C, will depend upon the tools at your service.
D is the front edge of an adjustable table, like that explained. (Index.) It is 4¼ in. wide. It supports the magnetic needle which is inside of E.
E is the outside of a glass-covered compass. ([See App. 67] for details.) The needle should not be over 1 in. long.