Owing to the astatic qualities of the needles, the base-block does not have to be turned so that the coil may face North and South, as in the current-detector. When the slightest current of electricity passes through the coil it instantly affects the needles, turning them to the right or left according to the way in which the current is running through the coil.
The sensitiveness of an astatic detector may be increased by the added strength of the coil-field for a given current.
There are two ways of accomplishing this result. The number of turns of wire may be increased in the coil, or two coils may be used, placed side by side. The latter method is the more satisfactory, since then the coil does not have to be opened at the top to admit the lower needle, the latter being dropped down between the coils. This apparatus is shown in the illustration of an astatic galvanometer, Fig. 8. The general arrangement of needles, bridge, and coils, is the same as described for the astatic current-detector.
Each coil is made separately of ten feet of No. 30 insulated copper wire, wound about the base of a drinking-glass to shape it; then pressed into elliptical shape, and fastened to a base-block with a brass or copper strip, and held down with small brass screws.
The base-block should be four inches square, with the corners sawed off. Smooth the block with sand-paper, and then give it several good coats of shellac.
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.