186. To Support the Needle, glue or sew two strips, I, to G. They must be in such a position that the poles of M will be as nearly as possible in a horizontal line drawn through the center of the circle, G. After you have made M ([App. 66]), and have found where the pieces, I, should be, fasten them to G, and then to I glue a pasteboard strip, J, about 1¼ in. wide. Run a pin, P, up through the center of J to support M.
187. The Magnetic Needle, M, should not be over 1 in. long for this kind of an instrument. ([See App. 66] for full directions for making it.) On the top of M should be fastened a light paper pointer or index, L. The short end should be made large, so that the long slim end will not over-turn M; that is, the pointer should balance itself. It may be fastened to M with paraffine or a drop of sealing-wax. If carefully balanced, the pointer can be made quite long.
188. The Graduated Circle, K, is described. (Index.) With this you can tell through how many degrees the needle is deflected, when the current passes. The strength of different currents can be compared, and many interesting experiments performed with the tangent galvanometer. For clearness, the circle, K, is shown small. In order to have the divisions on it far enough apart, K should be about 4 in. in diameter. The zero points should be at the front and back of the instrument, when a pointer is used on the needle.
189. How to Use It. For full explanations, and for the study of experimental cells, etc., by means of the tangent galvanometer, see text-book. It will be impossible for you to get M exactly in the center of G; you cannot get the pointer exactly at right angles with M; hence, if you pass a certain current through the coils, and the pointer reads 20 degrees, you will find, if you reverse the current, making it go through the coil in an opposite direction, that the pointer may read 24 degrees on the opposite side of the zero. To get the true reading, then, take the average of the two, which in the case mentioned would be 22 degrees. (See current reversers.)
190. Tangent Galvanometer. Fig. 91. The base consists of 2 parts, A and B. It is not necessary to use two pieces if you have wood that is at least ⅞ in. thick. This is given as a suggestion in case you have nothing but thin boards. By screwing B to A the base is made thick enough to take the screws for binding-posts. The base proper, A, is 8½ × 5 × ½ in. If you make this of ⅞ in. stuff, you will not need B.
The Back, C, is 10 × 8½ × ½ in. It is screwed to the base. Do not use nails, as these affect the magnetic needle. Find the center of C, and with this as a center, draw two circles, (that is, the circumferences of two circles,) one 5 in. in diameter to show where to cut out a hole, H, and the other 7 in. in diameter to serve as a guide for fastening on the spools, F.
