Apparatus. Same as in [Exp. 154]; a short rod or iron core, I C, of soft iron (No. 92) that will fit inside of the coil. This combination is called an electromagnet.
395. Directions. (A) Arrange first as for [Exp. 154], [Fig. 122], with the coil in the E and W line, no core being used, and place O C about 6 in. from the right-hand end of the coil.
(B) Press the lever for an instant to see whether the field of the coil is strong enough to move the compass-needle at that distance. Move O C a little nearer or farther from the coil until the needle just moves, when the circuit is closed.
(C) Place I C inside of the coil ([Fig. 125]), and repeat (B) to see whether the magnetic field of the coil is stronger or weaker than before.
(D) Study the location of the poles. Can they be reversed?
CHAPTER XXIII.
ELECTROMAGNETS.
396. Electromagnets are important to the student of electricity. They form the principal part of nearly every electrical instrument. You have seen that a wire has a magnetic field about it the instant a current passes through it. A coil, or helix of wire, has a stronger field than a straight wire carrying the same current, because each turn, or convolution, adds its field to the fields of the other turns. By having a core of soft iron instead of air, wood, or other non-magnetic material, the strength of the magnet is greatly increased. The central core may be permanently fixed in the coil, or it may be removable. (See Apparatus Book, Chapter IX, for Home-made Electromagnets.)
397. Cores of Electromagnets. A strong magnet has more lines of force passing from its N pole through the air to its S pole than a weak magnet. By increasing the number of lines of force we increase the strength of a magnet. It has been seen, in experiments with permanent magnets, that lines of force do not pass as readily through air as through soft iron, and that lines of force will go out of their way to pass through iron. It was learned in [Exp. 154] that inside of a helix ([Fig. 123]) the lines of force pass from the S to the N pole; they then spread out through the air and pass back on all sides of the coil to its S pole, as in the case of permanent magnets. The air around and inside of a helix offers a great resistance to the lines of force, and tends to weaken the[166] magnetic field. When part of the circuit consists of an iron core, which is a splendid conductor of lines of force, the magnetic field is greatly increased in strength.