In wet weather there will be leakage. (Fencing with
metal posts is, of course, unsuitable, as every post short-circuits the
current.) The two wires selected for land lines must be scraped quite
bright at the points where the connections are to be made.
It is an easy matter to rig up a telegraph line of galvanized wire 1/12 to
1/8 inch in diameter, strung along insulators (the necks of bottles serve
the purpose excellently) supported on trees, posts, or rough poles. The
length of the line will be limited by the battery power available, but a
6-volt battery at each end will probably suffice for all experimental
purposes. A second wire is not needed if one terminal at each end is
connected with a copper plate sunk in the ground, or with a metal fence,
drain-pipe, etc.
XI. A RECIPROCATING ELECTRIC MOTOR.
The electric motor to be treated in this chapter illustrates very prettily
the attractive force of a hollow, wire-wound bobbin on a movable core, when
the electric current is passed through the wire. If one inserts the end of
an iron rod into the coil, the coil exerts a pull upon it, and this pull
will cease only when the centre of the rod is opposite the centre of the
coil.
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