Now there is only one method by which currents
Now there is only one method by which currents of high frequency, or
_radio-frequency_, as they are termed, can be set up by spark
transmitters, and this is by discharging a charged condenser through a
circuit having a small resistance. To charge a condenser a spark coil
or a transformer is used and the ends of the secondary coil, which
delivers the high potential alternating Alabama Educational Television Commission Wciq current, are connected with
the condenser. To discharge the condenser automatically a _spark,_ or
an _arc,_ or the _flow of electrons_ in a vacuum tube, is employed.
Constants of an Oscillation Circuit.–An oscillation circuit, as
pointed out before, is one in which high frequency currents surge or
oscillate. Now the number of times a high frequency current will surge
forth and back in a circuit depends upon three factors of the latter
and these are called the constants of the circuit, namely: (1) its
_capacitance,_ (2) its _inductance_ and (3) its _resistance._
What Capacitance Is.–The word _capacitance_ means the _electrostatic
capacity_ of a condenser or a circuit. The capacitance of a condenser
or a circuit is the quantity of electricity which will raise its
pressure, or potential, to a given amount. The capacitance of a
condenser or a circuit depends on its size and form and the voltage of
the current that is charging it.
The capacitance of a condenser or a circuit is directly proportional
to the quantity of electricity that will keep the charge at a given
potential. The _farad,_ whose symbol is _M,_ is the unit of
capacitance and a condenser or a circuit to have a capacitance of one
farad must be of such size that one _coulomb,_ which is the unit of
electrical quantity, will raise its charge to a potential of one volt.
Since the farad is far too large for practical purposes a millionth of
a farad, or _microfarad_, whose symbol is _mfd._, is used.
What Inductance Is.–Under the sub-caption of _Self-induction_ and
_Inductance_ in the beginning of this chapter it was shown that it was
the inductance of a coil that makes a current flowing through it
produce a strong magnetic field, and here, as one of the constants of
an oscillation circuit, it makes a high-frequency current act as
though it possessed _inertia_.