Part 12 (1/2)

179. The Base should be about 7 5 7/8 in. Fasten three bent bra.s.s or copper strips to the base with bra.s.s screws to hold the chimney steady.

By bending them in more or less you can make a snug fit around the chimney.

180. Adjusting the Needle. In the center of the box-cover is a small hole. The thread from the needle pa.s.ses through this. The upper end of the thread is wound around a screw-eye, which is screwed into the cover near one edge. By turning the cover around, the needle can be made to hang parallel to the coils, and by turning the screw-eye, the needle can be raised or lowered. A small hole should be made in the cover before putting in the screw-eye, or you will be liable to split the wood.

[Ill.u.s.tration: Fig. 89.]

181. Use. This apparatus will indicate very slight currents; in fact, as feeble ones as the student will have occasion to experiment with, such as induced currents, currents of thermo-electricity, and currents produced by exceedingly weak batteries. (See text-book.)

APPARATUS 116.

_182. Tangent Galvanometer._ Fig. 90. For the uses of this form of galvanometer see text-book. Do not use any iron in making this apparatus. The base is 5 4 7/8 in. At its front end are three binding-posts. The pasteboard band, G, is 1-1/4 in. wide and 6 in. in diameter. Cut the pasteboard 21 in. long and 1-1/4 in. wide, then bend it into the form of a circle. There will be a lap of about 3 in., and you can make it solid by sewing the two ends together at the lap.

[Ill.u.s.tration: Fig. 90.]

183. The Coils maybe made of No. 24 insulated copper wire, which should be wound on before fastening G to the base. There are two separate coils, one having five turns and the other ten turns. Leaving a 6-in.

length, A, for connections, wind five turns of wire on to G, putting them on clockwise; that is, pa.s.s them over the top of G from left to right. Tie thread around G and the wire to hold them together after you have five turns on, and cut a 6-in. end, B. Now begin with C, and wind on ten turns, bringing the end of them out at D. Punch holes, F, through G on each side of the coils, run twine, T, through them, and tie T on the outside of G. Do this in three or four places, to firmly hold the coils.

184. Fastening Coils to Base. The band and coils will not rest squarely upon the base, so cut two pieces of wood, E, about 2 1/4 1/4 in., to be put under G, one being on each side of the coil. Make holes through the base, pa.s.s strong cord, H, through them, and over the inside of G, then tie under the base. This should tightly squeeze E, and hold G upright and firm.

185. The Connections. A and B are the ends of the five-turn coil; C and D are the ends of the ten-turn coil. If the battery-wires are connected with X and Y, the current will pa.s.s through five turns of wire; if connected with Y and Z, it will pa.s.s through ten turns; if with X and Z, the current will pa.s.s through the entire fifteen turns. In this way the strength of the magnetic field about the coil can be regulated, and its effect upon the magnetic needle, M, changed.

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 1-1/4 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 pa.s.ses. 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 pa.s.s 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.)

APPARATUS 117.

_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 7/8 in. thick. This is given as a suggestion in case you have nothing but thin boards. By s.c.r.e.w.i.n.g B to A the base is made thick enough to take the screws for binding-posts. The base proper, A, is 8-1/2 5 1/2 in. If you make this of 7/8 in. stuff, you will not need B.

The Back, C, is 10 8-1/2 1/2 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 circ.u.mferences 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.

[Ill.u.s.tration: Fig. 91.]

The Spools, F, are glued to C. If you have bra.s.s screws, these may be used instead of the spools; they should be left sticking out from C about 1 in. Around the spools or screws, fasten a pasteboard band, G, on which to wind the wire. G may be about 1 in. wide; it should be kept in the circular form by sewing the ends together where they lap. (Read directions in App. 116.)

191. The Coils on this model are 4 in number. (See App. 116 for the method of winding.) The first coil is made of coa.r.s.e wire, No. 18, its ends being joined to the binding-posts, V and W. The second coil has 5 turns of No. 24 insulated copper wire, its ends being joined to W and X.

The third coil has 10 turns of the same size wire, No. 24, and is joined to X and Y. The fourth coil has 20 turns of the same joined to Y and Z.

If you want to use the galvanometer for quite weak currents, it would be well to make a fifth coil of 20 turns of No. 30 wire, and join it with Z and a new binding-post. The ends of the coils are run through small screw-eyes before pa.s.sing to X, Y, etc. This is not necessary, it merely keeps them in place.

The Binding-Posts are like App. 43. Any other desired style may be used, those of App. 46 being preferred.

The Hole, H, is 5 in. in diameter. It should be cut out about 1/2 in.