Fig. 90 shows a bent wire through which a current passes from C to Z. If you look along the wire from C toward the points A and B, you will see that under the wire the lines of force pass to the left. Looking along the wire from Z toward D you will see that the lines of force pass opposite to the above, as the current comes toward you. This is learned by experiment. (See "Study," Exp. 152, § 385, etc.)
Fig. 90.
Fig. 91.
Rule. Hold the right hand with the thumb extended (Fig. 89) and with the fingers pointing in the direction of the current, the palm being toward the needle and on the opposite side of the wire from the needle. The north-seeking pole will then be deflected in the direction in which the thumb points.
93. Current Detectors. As there is a magnetic field about a wire when a current passes through it, and as the magnetic needle is affected, we have a means of detecting the presence of a current. When the current is strong it is simply necessary to let it pass once over or under a needle; when it is weak, the wire must pass several times above and below the needle, Fig. 91, to give the needle motion. (See "Apparatus Book," Chapter XIII., for home-made detectors.)
Fig. 92.