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.
The indicator-needle may be cut from stiff paper, thin sheet-fibre, or very thin cold-rolled brass or copper, the latter being commonly known as hard or spring-brass. Only one pointer is really necessary—that pointing to the front. But the weight of the material would have a tendency to upset the magnetic needle, and therefore it is better to carry an equally long tail or end, on the opposite side, to properly balance the needle.
A very weak current, passing in through the first post and out at the third, will cause the indicator to be deflected considerably, or so that it will point from 40° to 60° on either side of the zero point, according to the direction in which the current is running through the coils.
When not in use the magnetic needle should be removed from the pivot, and placed in a box or other safe place, where it will not become damaged.
A differently arranged tangent galvanometer is shown at [Fig. 14]. As the line of binding-posts would indicate, there are several coils of wire about the circle or hoop.
This galvanometer can be used for either strong or weak currents, since it is wound with both coarse and fine insulated wire. An upright plate of wood, seven inches wide and eight inches high, supports the hoop and compass. The top corners are sawed off, and four inches above the bottom a straight cut is made across the plate, five inches wide and arched in a half-circle five inches in diameter. A shelf of wood a quarter of an inch thick, three inches wide, and five inches long is made, and attached as a ledge in this arched opening, so that a compass three inches in diameter may rest upon it.