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.

APPARATUS 104.

_155. Interrupter._ Fig. 81. The body of this consists of a strip of wood, 6 or 7 in. long, 1-1/2 in. wide, and 7/8 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 1/4 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.

APPARATUS 105.

_156. Interrupter._ Fig. 82. The nails in this apparatus are placed in a circle about 4 in. in diameter. They are electrically connected to each other by a bare copper wire, which is wound around each nail several times, and then led out to one of the binding-posts. In the center of the circle is a nail, or screw, which is connected by a wire to the other binding-post, care being taken not to allow the two wires to touch each other. Around the central screw is wound one end of a stout wire, the other end of which reaches out from the screw far enough to touch the nails. When this stout wire touches any nail, a current entering one binding-post can pass through nails, screw, etc., and out at the other binding-post. When the end of the stout wire is between two nails, the current cannot flow. By placing the finger against this stout wire and turning it around rapidly, the current can be interrupted as desired. The base should be about 5 x 6 x 7/8 in.

APPARATUS 106.

_157. Interrupter._ Wind the end of the wire from one pole of the battery around the handle of the file. Scrape the other wire along the rough file. As it jumps from one ridge to another the current will be rapidly interrupted.

APPARATUS 107.

_158. Interrupter._ Hold the end of the wire from one pole of a battery upon a saw-blade. Draw the other wire along over the teeth of the saw. As the wire jumps from one tooth to the next the current will be broken.

APPARATUS 108.

_159. Automatic Interrupter._ An ordinary electric bell, or buzzer, may be used as an interrupter. Every time the vibrating armature swings, the circuit is opened. The combination of a battery, induction coil, and electric bell makes a very good outfit for medical purposes. The automatic interrupter used on App. 100 should be studied.