learly shown in connection with such
apparatus.

APPARATUS 105.

[Illustration: Fig. 82.]

_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.

CHAPTER XIII.

CURRENT DETECTORS AND GALVANOMETERS.

_160. Current Detectors; Galvanometers._ When a wire carrying a current
of sufficient strength is properly brought near a magnetic needle, the
latter will be deflected from its N and S line. The conducting wire has
a magnetic field while the current passes through it, and this gives the
wire the power to act upon a magnetic needle just as another magnet
would.

The action of detectors, etc., depends upon this fact; and, strange to
say, the magnetic field about the wire disappears the