392. Polarity of Coils. It is evident from Exps. 153 and 154 that a coiled conductor has poles, magnetic field, etc., when a current passes, and that it strongly resembles a magnet, even though no iron enters into its construction. We may say that the coil becomes magnetized by the electric current. [Fig. 123] shows a right handed coil or helix of wire, the current passing as shown by the small arrows. The[163] left-hand end is a S pole because the current passes around it in a clockwise direction. When you face the right-hand end of the coil the current is seen to pass around it in an anti-clockwise direction; this produces a N pole. As the N pole of the magnetic needle is attracted toward the S pole of the coil, it is clear that the lines of force pass through the inside of the coil as shown by the large arrows. They then curve through the air and return to the S pole as with magnets.
EXPERIMENT 155. To test the attracting and "sucking" power of a magnetized coil or helix.
Apparatus. The coil, battery, etc., used in [Exp. 154], [Fig. 122]; a sewing-needle.
393. Directions. (A) Arrange the coil, etc., as described in [Exp. 154]. The coil need not lie in the E and W line, however, and a key may be used instead of the current reverser.
(B) Magnetize the needle so that its point will be a N pole.
(C) Tie a thread about the center of the magnetized needle, hold the thread in the hand so that the S pole of the needle will swing freely at the hole at the right-hand end of the coil ([Fig. 124]). If the current passes as directed, the right-hand end of the coil will be a N pole. What happens to the needle when the key is pressed for an instant.
(D) Change the needle to the left end of the coil and repeat.