loops of the iron atoms so that more of them line up with the
turns of the primary. These atomic loops, of course, are turned by the
current in the primary. There are so many of them, however, that the
current due to their turning is usually the more important part of the
total current.

In all transformers the effect is greater the more rapidly the current
changes direction and the atomic loops turn around. For the same size of
electron stream in the primary, therefore, there is induced in the
secondary a greater e. m. f. the greater is the frequency with which the
primary current alternates.

Where high frequencies are dealt with it isn't necessary to have iron
cores because the effect is large enough without the help of the atomic
loops. And even if we wanted their help it wouldn't be easy to obtain,
for they dislike to turn so fast and it takes a lot of power to make
them do so. We know that fact because we know that an iron core
increases the inductance and so chokes the current. For low frequencies,
however, that is those frequencies in the audio range, it is usually
necessary to have iron cores so as to get enough effect without too many
turns of wire.

The fact that iron decreases the inductance and so seriously impedes
alternating currents leads us to use iron-core coils where we want high
inductance. Such coils are usually called "choke coils" or "retard
coils." Of their use we shall see more in a later letter where we study
radio-telephone transmitters.

LETTER 21

YOUR RECEIVING SET AND HOW TO EXPERIMENT

MY DEAR STUDENT:

In this letter I want to tell you how to experiment with radio
apparatus. The first rule is this: Start with a simple circuit, never
add anything to it until you know just why you are doing so, and do not
box it up in a cabinet until you know how it is working and why.

Your antenna at the start had better be a single wire about 25 feet high
and about 75 feet long. This antenna will have capacity of about 0.0001
m. f. If you want an antenna of two wires spaced about three feet apart
I would make it about 75 feet long. Bring down a lead from each wire,
twisting them into a pigtail to act like one wire except near the
horizontal part of the antenna.

[Illustration: Fig 110]

Your ground connection can go to a water pipe. To protect the house and
your apparatus from lightning insert a fuse and a little carbon block
lightning arrester such as are used by the telephone comp