_=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 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. =EXPERIMENTS 158-163. To study straight electromagnets.= _Apparatus._ A good dry cell or other source of a fairly strong current; coil with soft iron core; key; wires with connectors, etc.; small nails; iron-filings; compass; large wire nail; tin box (No. 94) to act as a base for the electromagnets. =EXPERIMENT 158. Lifting power.= =398. Directions.= (A) Join the cell, key, and coil, as explained in Exp. 154, so that the current will pass only when the key is pressed. Place the core inside of the coil (Fig. 125). Two good cells in series can be used to advantage. (B) Hold the coil in a vertical position near small nails, iron filings, tin boxes, etc.; then press the key and raise coil; carry the clinging iron to another place, break the circuit at the key, and explain the result. Why do nails cling more strongly to the core than filings after the circuit is broken? =EXPERIMENT 159. Residual magnetism of core.= =399. Directions.= (A) After the current has passed through the coil with the core in place, remove the core and test it for magnetism with the compass. Will the small end of the core attract both poles of the compass-needle, or is it slightly magnetized? (B) If there is any residual magnetism, strike the core with a hammer and test again. (C) Use a soft steel wire nail for the core, and repeat (A) and (B). Why does soft iron make a better core than steel for electromagnets? Which should be the more easily magnetized? =EXPERIMENT 160. Magnetic tick.=