EXPERIMENT 107. To study the construction and use of a simple astatic galvanoscope.
Apparatus. An astatic galvanoscope, A G (No. 59) ([§ 252]–[254]); dry cell, D C (No. 51); current reverser, C R (No. 57) ([§ 235]); wires for connections ([§ 226]).
Arrange as shown in [Fig. 80], which is a top view. The wires from C R are connected to the binding-posts of A G at the back, the spring connectors being slipped into them ([§ 229]).
252. Construction of the Astatic Galvanoscope. When not to be used for a long time, or for shipping, the legs, A ([Fig. 77]) may be removed, and the whole packed inside of the box, B.
The Coil, C, has a resistance of about 5 ohms, and is fastened to the coil-support, C S. The ends of the coil are permanently fastened to the binding-posts, L and R (left and right). The ends are so arranged that when the current enters at L it will pass around the coil in a clockwise direction.
253. The Astatic Needle ([Exp. 106]) is supported by a small thread, T, which is tied to the thread-wire, T W. This T W springs into an eyelet, E, which, in turn, rests in a hole made in the end of B. E should turn easily in the hole, but it should not wabble.
[Fig. 78] shows a sectional view of the coil and needle. The wire, W, should be bent, as shown, so that the magnets can be as near the center-line of C as possible. [Fig. 79] shows a front view of the needle. As a matter of convenience it will be best to arrange the poles of the needles, as shown, to agree with the descriptions of the experiments.
To keep the needle from being affected by air currents, the glass plate (No. 38) may be placed in front of the box, B. Stand it upon the legs, A, and tie a string around it, and B, to hold it in place.