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.)

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 screwing B to A the base is made thick
enough to take the screws for binding-posts. The base proper, A, is
8-1/2 x 5 x 1/2 in. If you make this of 7/8 in. stuff, you will not need
B.

The Back, C, is 10 x 8-1/2 x 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
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.

[Illustration: Fig. 91.]

The Spools, F, are glued to C. If you have brass screws, these may be