The bridge is made from brass one-sixteenth of an inch thick and half an inch wide. The coils of wire are arranged about half an inch apart, and at both ends a small separator-block is placed between the coils, and then bound with silk or cotton thread. A circular indicator disk of bristol-board should be cut out and marked and attached to the top of the coils with a few drops of sealing-wax or paraffine; then the needles are suspended so as to hang properly, one above the card, the other between the coils.
Three binding-posts are placed at one end of the block, and to them the end wires of the coils are led and attached. To the first binding-post (at the left) the strand of wire leading to the first coil is attached. It leads in and is coiled as the hands move on a clock, from left to right. The leading-out wire from the coil is made fast to the middle post. The leading-in wire to the second coil is also made fast to the middle post. The coil wires should have the turns in the same direction as the first coil; then the last wire is attached to the right-hand post.
When making connections for a strong current, use an end and middle post. This arrangement will operate but one coil. For very weak currents make the leading in and out wires fast to the end-posts. This latter plan is more clearly shown in the diagram, [Fig. 9]. A and B represent the coils, C, D, and E the binding-posts. The current, entering at C, passes through the coil A (as the hands move about the dial of a clock) and out at D, where connection is made with the wire leading in to coil B. The current passes through this coil in the same direction as the clock hands move, and out to post E. Be careful to arrange the wiring and connections after this exact manner, otherwise the instrument will not be of any use.
The adjustment at the top of the bridge may be made with an inverted screw-eye and a small cork into which the eye can be screwed, thus raising or lowering the needles to the proper position. Be sure to have the needles in parallel when at rest.
As the needles and coils are very sensitive it would be well to cover the instrument with an inverted glass jar. A bluestone or gravity battery jar will answer very well, and after the wires are connected to the binding-posts the glass may be placed over the entire apparatus.
A Tangent Galvanometer
For testing the various degrees of intensity of a current a tangent galvanometer is usually employed. In this apparatus the increased strength is indicated by the index-pointer as it plays over a scale or graduated circle.
A simple tangent galvanometer may be made from a flat hoop of wood-fibre or brass, mounted on a base by means of two uprights, together with the necessary compass needle, an index-card, insulated wire, and binding-posts for the electrical connections. This piece of apparatus is shown in [Fig. 10]. It is built on a base-block six by seven inches and three-quarters of an inch thick. The block should be of selected wood, and after it is made smooth it should be given several coats of shellac.
Two upright pieces of wood, five inches long, half an inch thick, and one inch in width, are screwed fast to the rear edges of the base-block to support the hoop on which the insulated wire is wound. Be careful not to use any iron or steel in the construction of this or any other recording instrument, except where it is expressly stated. Screws, nails, staples, or any bits of anchoring wire should be of copper or brass. String, thread, or silk may be used, especially where coils of wire are to be bound or fastened to hoops or base-blocks. The balance of the indicating needle is so delicate, and the sensitiveness of the coils is so easily affected, that nothing about or near the instruments should be of iron or steel.
The hoop may be made of very thin hickory wood, steamed and bent so as to form a ring six inches outside diameter and one inch wide. It is even possible to construct a satisfactory hoop from a ribbon of brown paper, rolled and lapped, the several thicknesses being glued as the turns are made.