(B) Remove the connector from Cu, swing it over into the position of the dotted line (Fig. 115), slip the connector upon M P and watch the needle. This cuts the cells out of the circuit; but, if you desire, also remove wire 3 from M P. Does the storage cell, S C, produce any current? Does it pass through A G in the same direction as that which came directly from the two cells? (C) Try the dry cell in place of the two simple cells. Try 2 other cells in series if you have them. _=382. Secondary or Storage Cells=_ must be charged by a current before they can give out a current. _Electricity_ is not really stored. Chemical changes are produced in the storage cell by the charging current, as in the voltameter or electroplating bath; and it is, then, potential chemical energy that is stored. When the new compounds are allowed to go back to their original condition by joining the electrodes of the charged cell a current is produced. In other words, an electric current produces chemical changes in the cell by electrolysis, and these new compounds have an E. M. F. of polarization because they are constantly willing and anxious to get back to their old state. The plates are lead and are usually coated with compounds of lead. Hydrogen and oxygen are given out at the electrodes. The current from a dynamo is used to charge secondary batteries. (See "Things a Boy Should Know About Electricity.") CHAPTER XXII. ELECTROMAGNETISM. _=383. Electromagnetism=_ is the name given to magnetism that is developed by electricity. You have already seen that if a magnetic needle be placed in the magnetic field of a _magnet_, its N pole will point in the direction in which the lines of force pass on their way from the N to the S pole of the magnet. You have also seen that in the galvanoscope, etc., a coil of wire acts like a magnet when a current passes through it. Can we not, then, use the needle to study the lines of force about wires and coils? [Illustration: Fig. 116.] [Illustration: Fig. 117.] =EXPERIMENT 152. To study the lines of magnetic force about a straight wire carrying a current.= _Apparatus._ The compass, O C; key, K; dry cell, D C. Arrange as in Fig. 116. =384. Directions.= (A) Arrange the wire so that the current will flow through it from N to S over the compass-needle as soon as the circuit is closed (Fig. 117, A). Press K for an instant only, and note the direction in which the N pole is deflected. Repeat two or three times until you get clearly in mind the direction taken by the needle. Sketch the result in your note-book, and compare with Fig. 118, A. The arrow shows the direction of the current. (B) Let the current pass for an instant from N to S and _under_ the needle, as shown in Fig. 117, B. Sketch result. (C) Let the current pass for an instant from S to N _above_ the needle (Fig. 117, C). Sketch result.