Make a bridge or standard in the shape of an inverted U out of thin wooden strips and fasten it to the block.
The needles are ordinary sewing-needles which have been magnetized and shoved through a small carrier-bar, made from a strip of cardboard, with their poles opposite one another, as shown in the illustration.
Fig. 113.—Astatic Needles.
They may be held in place in the cardboard strip by a small drop of sealing-wax.
A small hole is punched in the top of the carrier, through which to pass the end of a thread. The upper end of the thread passes through a hole in the bridge and is tied to a small screw-eye in the center of the upper side of the bridge.
The carrier-bar is passed through the space where the coil is split at the top. The lower needle should hang in the center of the coil. The upper needle should be above and outside the coil.
The terminals of the coil are connected to two binding-posts mounted on the base-block.
Owing to the fact that this galvanoscope is fitted with an astatic needle, the instrument does not have to be turned so that the coil may face North and South. The slightest current of electricity passing into the coil will instantly affect the needles.
An astatic galvanometer for the detection of exceedingly weak currents and for use in connection with a "Wheatstone bridge" for measuring resistance, as described farther on, will form a valuable addition to the laboratory of the boy electrician.