243. The Compass-needle, furnished with O C (No. 18), will do also for this galvanoscope. It should be placed upon the pin-point after fixing on the pointers ([§ 246]). The length of the needle should be parallel to the plane of the coil when no current passes; that is, the coil and coil-support should be in the N and S line.
The needle can be centered in regard to right and left, and in regard to up and down, by properly adjusting the position of the pin-point support, P P S; this is held firmly to C S by two spring-connectors. By removing S C, the support, P P S, may be raised or lowered.
244. To place the coil in the N and S line, simply swing the galvanoscope bodily around, at the same time looking down upon the needle, until the length of the needle becomes parallel to the coil-support. When once carefully adjusted N and S, a line may be drawn upon the table as a guide for its position in future experiments. The coil should stand in a vertical plane, and this straight up and down position can be easily adjusted.
To place the coil in the E and W line, turn it until the pointers are at the 90° (90 degree) marks,—the 0° (zero degree) marks remaining, of course, as described above.
245. The Degree-Card, G D C ([Fig. 72]) has a dot at its center, to show where to make a pin-hole for the pin that supports the compass-needle. With this you can tell how many degrees the needle is deflected when the current passes. This card, G D C, should be pressed down over the pin-point. The zero points of G D C should be N and S, also, when the coil is in that position; that is, they should be in the plane of the coil. The pointers on the needle ([§ 246]) will then be at O, when the needle is at rest, no current passing through the coils. (See Apparatus Book § 272 for Home-Made Degree-Card.) G D C may be held permanently in position after it is adjusted, by sticking a short pin through it into P P S. Do not let this pin interfere, however, with the swinging of the needle.
246. Pointers ([Fig. 73]) should be fastened to the needle, in order to make the readings of degrees accurate. Fasten to the compass-needle a piece of No. 30 insulated copper wire, as shown. It may be cut to the proper length after it is wound around the needle. See that the wire does not touch the pin when needle is in place; balance needle by cutting a little from the heavier end of wire with shears; bend the ends of wire so that they are at opposite sides of the degree-card, both pointing at O, for example. The needle must swing freely, be nicely balanced, and the wire must not touch pin or degree-card.
