168. The Coil is wound around 2 ordinary spools which are glued to a vertical piece, which, in turn, is screwed to a base. You should not use iron nails or screws in the construction of electrical apparatus, when a magnetic needle is to be used in connection with it, as these would attract the needle. The spools may be pushed onto dowels which are fastened into the vertical piece. Small brass screws are good for the purpose also, if you haven't good glue or the dowels. This coil, etc., may be used in connection with an astatic needle. The coil may be wound with [App. 93] or [94], if you make the attachment of [App. 95], and screw the upright carrying the spools to the attachment.
The binding-posts, shown in Fig. 85, are not to be advised. It will be better to use those of [App. 45]. The magnetic needle is supported by a sewing-needle stuck through a cork. This may be fastened to the base with paraffine.
169. It is often troublesome to turn the apparatus around until the needle becomes parallel to the length of the coil. To avoid this, a small bar magnet, shown in the Fig. 85, may be laid on top of the coil. A magnetized sewing-needle will do, and this will keep the magnetic needle quiet and parallel to it when the current is not passing through the coil. Of course, it takes a little more current to move the magnetic needle when the bar magnet is in place, than it does without the magnet.
170. By allowing the current to enter the right-hand binding-post, as you look at it from the front (Fig. 85), it will go around the coil in the direction of the hands of a clock, that is, from left to right on top. This, of course, is not necessary to merely detect the presence of a current. In order, however, to determine the direction of currents by means of a magnetic needle, study the effect with a single turn of wire at first. (See text-book.)
171. Dimensions. The base is 5 × 4 × ⅝ in. The upright piece is 5 × 3½ × ⅝ in. The spools are 2½ in. apart center to center.
Fig. 86.
172. Astatic Current Detector. Fig. 86. The ordinary magnetic needle points to the north quite strongly. It is evident, then, that this pointing-power must be overcome by the magnetic field around the coil of wire, before the needle can be forced from the N and S line. Very weak currents will not visibly move the magnetic needle in the detectors so far described. You should remember that no action will take place unless the magnetic field around the magnetic needle is acted upon by that around the coil. In order to make an instrument that will be very sensitive, we must have strong fields about the needle and coil, and we must, at the same time, decrease the pointing-power of the needle. We can increase the strength of the field about the needle, and at the same time decrease its pointing-power by using an astatic needle. ([See App. 69].) The arrangement shown in Fig. 86 is a very simple one, and it is quite sensitive.