The word “astatic” means having no magnetic directive tendency. If the needles of this astatic pair are separated and pivoted each will point to North and South, after the ordinary fashion. For all astatic instruments we must employ two magnetic needles in parallel, either side by side or one above another, as shown in Fig. 6, with the N and S poles arranged as indicated. This combination is usually called Nobili’s pair. If both needles are of equal length and magnetic strength, they will be astatic, for the power of one counterbalances that of the other. As a consequent neither points to North. A compound needle of this form requires but a very feeble current to turn it one way or the other, and this is the theory upon which all astatic instruments are constructed. A simple astatic current-detector may be made from a single coil of fine insulated wire, a pair of magnetic needles, and a support from which to suspend them, together with a base-block. For the base-block obtain a piece of white-wood, pine, or cypress, four inches square and three-quarters of an inch thick. Sand-paper it smooth, and then give it two or three coats of shellac. From a strip of copper or brass (do not use tin or iron) make a bridge, in the form of an inverted [V], seven inches high, using metal one-sixteenth of an inch thick and half an inch wide. This bridge is to be screwed to the outside of the block, as shown at Fig. 7, so that it will be rigid and firm. A small hole is drilled through the top of the bridge to admit a screw-eye for the tension. Make a coil of No. 30 insulated wire, using ten or fifteen feet, and wind it about the base of a drinking-glass to shape it; then remove it and tie the coil, in several places, with cotton or silk thread, so as to hold the strands together. Shape it in the form of an ellipse and make it fast to the middle of the base-board with small brass or copper straps, and copper tacks or brass screws. Be very careful not to use iron, steel, or tin about this instrument, as the presence of these metals would deflect the needles and make them useless. Separate the strands at the top of the coil so that one of the needles may be slipped through to occupy a position in the middle of the coil. Ordinary large compass needles may be employed for this apparatus, or magnetized pieces of highly tempered steel piano-wire will answer just as well. [Illustration: FIG. 6] [Illustration: FIG. 7] [Illustration: FIG. 8]