_175. Astatic Detector._ Fig. 88. As previously Stated, the sensitiveness of a detector can be made greater by increasing the strength of the coil-field for a given current. This may be done by increasing the number of turns of wire in the coil. The most convenient way will be to use two coils, one on each side of the astatic needle.
176. The Support, or framework, is a lamp chimney. By this the astatic needle is suspended and protected from air currents. The chimney should be at least 3 in. in diameter at the bottom, about 10 in. high, with a plain round top. Upon the top of the chimney is placed the cover of a wooden pill-box, 2 in. in diameter.
177. The Coils should be made separately, for convenience. Each should be of 10 ft. No. 30 wire. (See details § 163.) Cut out a round piece of stiff pasteboard, just large enough to go inside of the bottom of the chimney. Fasten the coils to this by sewing (§ 163), or with paraffine, so that they shall be symmetrically located and 3/8 in. apart. The pasteboard circle may be fastened to the base with small brass screws. Do not use any iron nails or tacks. In this, all four ends of wire are brought out under the edge of the chimney (Fig. 88). Cut little grooves in the base for the wire to sink into, so that the chimney will rest firmly upon the base all around. The ends of the wires are fastened to three binding-posts.
178. Joining the Coils. The end of one coil must be joined to the beginning of the other properly, or the action of one will destroy that of the other. Fig. 89 shows the two coils, A and B. If the current enters at the binding-post, X, it will pass through the turns of coil A, in the direction of clock-hands, then out to Y, where B begins, around B in the same way, and then to Z. Y may be simply a screw-eye binding-post (App. 41). By this arrangement one or both coils can be used at a time. If the current is very weak, use both coils; that is, connect the ends of wires to be tested with the two outside binding-posts. If they are joined to the middle and one outside post, one coil only will be in the circuit.
179. The Base should be about 7 × 5 × 7/8 in. Fasten three bent brass or copper strips to the base with brass screws to hold the chimney steady. By bending them in more or less you can make a snug fit around the chimney.