Harper's Electricity Book for Boys

by Joseph H. Adams

A short piece of brass, copper, or wood will act as the carrier-bar for the needles. These should be pushed through holes made in the bar, and held in place with a drop of shellac or melted paraffine. A small hole is drilled at the top of the bar, or a small eye can be attached, through which to pass the end of a thread. The upper end of the thread is tied in a screw-eye, the screw part of which passes up through the hole in the bridge and into a wooden button or knob, which can be turned to shorten or lengthen the thread, and so raise or lower the needles. The lower needle must be pivoted at an equal distance between the upper and lower parts of the coil.

Two binding-posts are arranged at the corners of the base, and the ends of the coil wires are attached under the screw-heads. The in-and-out wires are to be made fast under the copper washers on the screw-eyes.

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.

An Astatic Galvanometer

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.