want to, you may take the cigar-box telegraph out into
the yard, leaving the batteries in the laboratory, while you
try to telegraph this short distance.

Examine a regular telegraph instrument. Trace the wire from
one binding post, around the coil and through the key, back
to the other binding post, and notice how pushing down the
key completes the circuit and how raising it up breaks the
circuit.

[Illustration: FIG. 140. Connecting up a real telegraph instrument.]

EXPERIMENT 77. Connect two regular telegraph instruments,
leaving one at each end of the long laboratory table. Make the
connections as follows:

Take a wire long enough to go from one instrument to the
other. Fasten the bare ends of this wire into the right-hand
binding post of the instrument at your left, and into the
left-hand binding post of the instrument at your right; that
is, connect the binding posts that are nearest together, as in
Figure 141.

Now connect one wire from the laboratory battery to the free
post of the right-hand instrument. Connect the other wire
from the laboratory battery to the ground through a faucet,
radiator, or gas pipe, making the connection firm and being
sure that there is a good, clear contact between the bare end
of the wire and the metal to which the wire is attached.

Make another ground connection near the left-hand instrument;
that is, take a wire long enough to reach from some pipe or
radiator to the left-hand telegraph instrument, bind one bare
end of this wire firmly to a clean part of the pipe and bring
the other end toward the instrument. Before attaching
this other end to the free binding post of the left-hand
instrument, be sure to open the switch beside the telegraph
key by pushing it to your right. Close the switch on the
other instrument. Now attach the free ground wire to the free
binding post of your telegraph instrument, and press the key.
Does the other instrument click? If not, disconnect the ground
wire and examine all connections. Also press the sounder of
each instrument down and see if it springs back readily. It
may be that some screw is too tight, or too loose, or that a
spring has come off; tinker awhile and see if you cannot make
the instrument work. If you are unable to do so, ask for help.

Figure 141 is a diagram of all the con