ith water, L is a lamp chimney. One wire, A, is fastened to T directly,
or by a spring binding-post. The other wire, B, is fastened to a piece
of copper, C, which may be raised or lowered inside of L. D is a piece
of pasteboard with a small hole in its center.

153. Use. If this apparatus be put anywhere in the primary circuit, the
amount of shock can be regulated by raising or lowering C. When C is
raised, the current has to pass through a longer column of water than it
does when C is near the bottom of L. When C touches T, the current
passes easily. If it were not for the chimney, the current would pass to
the sides of T.

[Illustration: Fig. 80.]

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.

[Illustration: Fig. 81.]

_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 c