[Footnote 3: A mil-ampere is a thousandth of an ampere just as a
millimeter is a thousandth of a meter.]

LETTER 10

CONDENSERS AND COILS

DEAR SON:

In the last letter we learned of an alternating e. m. f. The way of
producing it, which I described, is very crude and I want to tell how to
make the audion develop an alternating e. m. f. for itself. That is what
the audion does in the transmitting set of a radio telephone. But an
audion can't do it all alone. It must have associated with it some coils
and a condenser. You know what I mean by coils but you have yet to learn
about condensers.

A condenser is merely a gap in an otherwise conducting circuit. It's a
gap across which electrons cannot pass so that if there is an e. m. f.
in the circuit, electrons will be very plentiful on one side of the gap
and scarce on the other side. If there are to be many electrons waiting
beside the gap there must be room for them. For that reason we usually
provide waiting-rooms for the electrons on each side of the gap. Metal
plates or sheets of tinfoil serve nicely for this purpose. Look at Fig.
25. You see a battery and a circuit which would be conducting except for
the gap at _C_. On each side of the gap there is a sheet of metal.
The metal sheets may be separated by air or mica or paraffined paper.
The combination of gap, plates, and whatever is between, provided it is
not conducting, is called a condenser.

Let us see what happens when we connect a battery to a condenser as in
the figure. The positive terminal of the battery calls electrons from
one plate of the condenser while the negative battery-terminal drives
electrons away from itself toward the other plate of the condenser. One
plate of the condenser, therefore, becomes positive while the other
plate becomes negative.

[Illustration: Fig 25]

You know that this action of the battery will go on until there are so
many electrons in the negative plate of the condenser that they prevent
the battery from adding any more electrons to that plate. The same thing
happens at the other condenser plate. The positive terminal of the
battery calls electrons away from the condenser plate which it is making
positive until so many electrons have left that the protons in the atoms
of the plate are calling for electrons to stay home just as loudly and
effectively as the positive battery-terminal is calling them away.

When both these conditions are reached--and they are bot