NT 75. Bare an inch of each end of a piece of
insulated wire about 10 feet long. Fasten one end to the zinc
of your battery or to one wire from the storage battery; wrap
the wire around and around an iron machine bolt, leaving the
bolt a foot or so from the battery, until you have only about
a foot of wire left. Hold your bolt over some iron filings. Is
it a magnet? Now touch the free end of your wire to the carbon
of your battery or to the other wire from the storage battery,
and hold the bolt over the iron filings. Is it a magnet now?

You have completed the circuit by touching the free end of
the wire to the free pole of your battery; so the electricity
flows through the wire, around the bolt, and back to the
battery.

Disconnect one end of the wire from the battery. You have now
broken the circuit, and the electricity can no longer flow
around the bolt to magnetize it. See if the bolt will pick
up the iron filings any more; it may keep a little of its
magnetism even when no electricity is flowing, but the
magnetism will be noticeably less. When you disconnect the
wire so that the electricity can no longer flow through a
complete circuit from its source back to its source again, you
are said to _break the circuit_.

[Illustration: FIG. 139. Sending a message with a cigar-box
telegraph.]

EXPERIMENT 76. Examine the cigar-box telegraph (see Appendix
B) and notice that it is made on the same principle as was the
magnetized bolt in Experiment 75. Complete the circuit through
the electromagnet (the bolt wound with wire) by connecting
the two ends of the wire that is wrapped around the bolt,
with wires from the two poles of the battery. By making and
breaking the circuit (connecting and disconnecting one of the
wires) you should be able to make the lower bolt jump up
and down and give the characteristic click of the telegraph
instrument.

In this experiment it does not matter how long the wires are
if the batteries are strong enough. Of course it makes no
difference where you break the circuit. So you could have the
batteries in the laboratory and the cigar box a hundred miles
away, with the wire going from the batteries to the bolt and
back again. Then if you made and broke the circuit at the
laboratory, the instrument would click a hundred miles away.
If you