CHAPTER XII.
CONTACT BREAKERS AND CURRENT INTERRUPTERS.
154. Contact Breakers; Current Interrupters. It is often necessary to make and break the electric current at frequent intervals. This can be done by an ordinary key ([App. 118]) by rapidly raising and lowering it. It is more convenient, however, to use some other form of apparatus. The current may be interrupted automatically; that is, it may be made to do the work itself ([App. 100]), or each make and break in it may be governed by the student.
Fig. 81.
155. Interrupter. Fig. 81. The body of this consists of a strip of wood, 6 or 7 in. long, 1½ in. wide, and ⅞ in. thick. Cut a strip of tin 1 in. wide and long enough to bend down over the ends of the wood. Fasten the tin to the wood with small wire nails, driving the nails into the ends as well as into the top of the strip. Make a "center line" along the tin as a guide, and then drive 1-in. wire nails through the tin into the wood, so that they will make a row the length of the wood, and stand about ¼ in. apart. On one end make a hole through the tin, and put in a screw-eye binding-post ([App. 45]). It is evident that if a wire from one pole of a battery be connected with the binding-post, it will also be electrically connected with the tin strip and nails. By touching the wire from the other battery-pole to the tin or to any nail, the circuit will be closed. If this last-mentioned wire be drawn along entirely above the tin, so that its end can bump along from one nail to another, you can see that the current will be closed every time a nail is touched, and be opened every time it jumps through the air. This apparatus can be connected with shocking coils, induction apparatus, etc., etc. Its use will be more clearly shown in connection with such apparatus.