=469. Directions.= (A) Cut from a tin box, or can, a piece of tin about 4 in. long and 1-1/2 in. wide. Bend this double to make two thicknesses. This will serve as an armature I (Fig. 157). Nail to one end of I a small spool, S, and into this put a short length of lead-pencil, P, which may be held firmly in S by wrapping a little paper around it. Connect the ends of coil H to a key and cell as in Fig. 156. (B) Hold or fasten I in place, and have a friend make dots and dashes at the key, while you draw a piece of paper past the end of P. A little adjusting will be necessary to get the pencil to write only while the circuit is closed. In regular machines all the parts are automatic. [Illustration: Fig. 158.] =EXPERIMENT 198. To study the action of a simple "annunciator."= =470. Directions.= (A) Arrange as in Fig. 158. Fasten the two electromagnets, H and E, to a board or a piece of stiff cardboard. They may be held in place by passing strings over them and through the board, tying on the other side. The ends of coils H and E should be joined to pieces of tin, A, B, C, by means of connectors. K and K are keys or push-buttons, which in real instruments are in different rooms. Two steel pens may be swung on pins a short distance from the ends of the cores, so that their lower ends will be attracted to the cores the instant the current passes through them. The residual magnetism should hold them against the cores until removed. Hairpins, nails, or needles can be used instead of pens. (B) Press first one K and then the other to see whether your connections are correct. _=471. Annunciators.=_ There are many forms of annunciators in use to indicate, in a hotel for example, a certain room when a bell rings at the office. If a bell be included in the circuit between D C and A in Fig. 158, it will ring each time a key is pushed. This will call attention to the fact that some one has rung, and the annunciator will show the location of the special call. Large instruments are made with hundreds of electromagnets, each one answering to a special room. The instrument should be set, of course, after each call. A nail or screw wound with insulated wire can be used for the electromagnets of a home-made annunciator. =EXPERIMENT 199. To study the shocking effects of the "extra current."= =472. Directions.= (A) Use the two electromagnets joined to the connecting plates (Fig. 132), to generate a self-induced or extra current. Connect R of Fig. 132 with the zinc of a dry cell, and between L and the carbon of the cell place a key; in other words, join the electromagnets, cell, and key in series. Two good cells in series can be used to advantage.