EXPERIMENT 106. To study the construction and use of a simple astatic needle.
Apparatus. Two unmagnetized sewing-needles (No. 1); horseshoe magnet, H M (No. 16); piece of stiff paper doubled and cut as in [Fig. 75]; a pin-point on which to support the paper. The pin may be stuck through a cork, or that of O C (No. 18) may be used.
250. Directions. (A) Draw each needle across the N pole of H M five times from point to head ([Exp. 9]). This should make them of nearly equal strength, both points being N poles.
(B) Stick the needles through the paper as shown, the N poles being at the same end of the paper. Balance the paper upon the pin-point. Has this combination a strong or weak pointing-power?
(C) Turn one of the needles end for end. Again test the pointing-power.
251. Discussion; Astatic Needles. By arranging the needles so that their poles oppose each other, the pointing-power becomes almost nothing. This sort of a needle is needed in some experiments in electricity. Their magnetic fields are still retained. The combination is called an astatic needle; it is used to detect very feeble[99] currents. The more nearly equal the magnets are in strength, the better. They are usually arranged with one above the other ([Fig. 76]).
