Kinds of Transmitters.--There are two general types of transmitters
used for sending out wireless messages and these are: (1) _wireless
telegraph_ transmitters, and (2) _wireless telephone_ transmitters.
Telegraph transmitters may use either: (a) a _jump-spark_, (b) an
_electric arc_, or (c) a _vacuum tube_ apparatus for sending out dot
and dash messages, while telephone transmitters may use either, (a) an
_electric arc_, or (b) a _vacuum tube_ for sending out vocal and
musical sounds. Amateurs generally use a _jump-spark_ for sending
wireless telegraph messages and the _vacuum tube_ for sending wireless
telephone messages.

The Spark Gap Wireless Telegraph Transmitter.--The simplest kind of a
wireless telegraph transmitter consists of: (1) a _source of direct or
alternating current_, (2) a _telegraph key_, (3) a _spark-coil_ or a
_transformer_, (4) a _spark gap_, (5) an _adjustable condenser_ and
(6) an _oscillation transformer_. Where _dry cells_ or a _storage
battery_ must be used to supply the current for energizing the
transmitter a spark-coil can be employed and these may be had in
various sizes from a little fellow which gives 1/4-inch spark up to a
larger one which gives a 6-inch spark. Where more energy is needed it
is better practice to use a transformer and this can be worked on an
alternating current of 110 volts, or if only a 110 volt direct current
is available then an _electrolytic interrupter_ must be used to make
and break the current. A simple transmitting set with an induction
coil is shown in Fig. 2.

[Illustration: Fig 2.--Simple Transmitting Set.]

A wireless key is made like an ordinary telegraph key except that