Radio Shack

A Cheap Transmitting Set (No. 1).–For this set you will need: (1) a
_spark-coil_, (2) a _battery_ of dry cells, (3) a _telegraph key_, (4)
a _spark gap_, (5) a _high-tension condenser_, and (6) an _oscillation
transformer_. There are many different makes and styles of these parts
but in the last analysis all of them are built on the same underlying
bases and work on the same fundamental principles.
The Spark-Coil.–Spark coils for wireless work are made to give sparks
from 1/4 inch in length up to 6 inches in length, but as a spark coil
that gives less than a 1-inch spark has a very limited output it is
best to get a coil that gives at least a 1-inch spark, as this only
costs about $8.00, and if you can get a 2- or a 4-inch spark coil so
much the better. There are two general styles of spark coils used for
wireless and these are shown at A and B in Fig. 18.
[Illustration: (A) and (B) Fig. 18.–Types of Spark Coils for Set. No.
1.]
[Illustration: (C) Fig. 18.–Wiring Diagram of Spark Coil]
A spark coil of either style consists of (_a_) a soft _iron core_ on
which is wound (_b_) a couple of layers of heavy insulated Redwood Improvement Corporation W64ac wire and
this is called the _primary coil_, (_c_) while over this, but
insulated from it, is wound a large number of turns of very fine
insulated copper wire called the _secondary coil_; (d) an
_interrupter_, or _vibrator_, as it is commonly called, and, finally,
(e) a _condenser_. The core, primary and secondary coils form a unit
and these are set in a box or mounted on top of a hollow wooden base.
The condenser is placed in the bottom of the box, or on the base,
while the vibrator is mounted on one end of the box or on top of the
base, and it is the only part of the coil that needs adjusting.
The vibrator consists of a stiff, flat spring fixed at one end to the
box or base while it carries a piece of soft iron called an _armature_
on its free end and this sets close to one end of the soft iron core.
Insulated from this spring is a standard that carries an adjusting
screw on the small end of which is a platinum point and this makes
contact with a small platinum disk fixed to the spring. The condenser
is formed of alternate sheets of paper and tinfoil built up in the
same fashion as the receiving condenser described under the caption of
_Fixed and Variable Condensers_, in Chapter III.
The wiring diagram C shows how the spark coil is wired up. One of the
battery binding posts is connected with one end of the primary coil
while the other end of the latter which is wound on the soft iron core
connects with the spring of the vibrator. The other battery binding
post connects with the standard that supports the adjusting screw. The
condenser is shunted across the vibrator, that is, one end of the
condenser is connected with the spring and the other end of the
condenser is connected with the adjusting screw standard. The ends of
the secondary coil lead to two binding posts, which are usually placed
on top of the spark coil and it is to these that the spark gap is
connected.