=EXPERIMENT 156. To find whether a piece of steel can be permanently magnetized by an electric current.= _Apparatus._ Same as for last experiment; an unmagnetized sewing-needle; the compass. =394. Directions.= (A) Be sure that the needle is not magnetized. It should attract both ends of the compass-needle. How can any magnetism in the needle be removed? (B) Place the needle inside of the coil with its _point_ to the east; that is, with its point at the N pole of the coil, and its head at the S pole. Close the circuit for an instant. Test the needle again for poles. Is the point a N or a S pole? (C) Turn the needle end for end in the coil, and see whether its polarity can be reversed. (D) Experiment with iron wire, nails, steel pens, spring steel, etc. [Illustration: Fig. 125.] =EXPERIMENT 157. To study the effect of a piece of iron placed inside of a magnetized coil of wire.= _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.)