For ease in transport it will be found advisable to mount key, buzzer, and battery on a common baseboard, which should be provided with a cover and handle.  The three parts are interconnected with one another, and the line wire terminals as sketched in Fig. 34.  This arrangement makes the apparatus very compact and self-contained.  As a finishing touch fit the lid inside with clips for holding a stiff-backed writing pad and pencil for the recording of messages.

Lines.—­Fencing made of stout galvanized iron wires strung on wooden posts supplies excellent conductors for practice purposes, provided the posts be quite dry.  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.  This principle is used in the “electric gun,” which in its simplest form is merely a series of powerful coils arranged one behind another on a tube through which an iron or steel projectile can pass.  The projectile closes automatically the circuit of each coil in turn just before reaching it, and breaks it before its centre is halfway through the coil, being thus passed along from one coil to the other with increasing velocity.

Our motor is essentially a very inefficient one, its energy being small for the current used, as compared with a revolving motor of the usual kind.  But it has the advantage of being very easy to make.

[Illustration:  Fig. 37.—­Electric reciprocating engine and battery.]