The oscillator or spark gap is one of the most important yet often the
most poorly adjusted part of a wireless station.
To obtain a good oscillatory discharge with little damping it is
necessary that the resistance of the circuit should be kept low, and
since the greatest part of the resistance is in the spark gap it would
appear as if this must be very short. While there are reasons for
keeping it short there are on the other hand very good reasons why it
should not be made too short, and the proper length should be a sort of
compromise to be determined by experiment.
If the gap is too short, the discharge will form an arc and the only
oscillations taking place will be those corresponding to the frequency
of the charging current. Power consumption is also in favor of a long
gap, since it causes the induction coil or transformer to draw less
current from the line. Another argument in favor of a long gap is the
fact that the condenser is charged to a higher voltage and more energy
stored up, which makes a greater distance of transmission possible. But
as stated above (and there always is a tendency for the amateur operator
to open out his gap as long as the sparks will continue to jump
steadily) the discharge will not oscillate but will merely set up a
unidirectional current.
There exists a proper gap length for a given circuit which will cause
that circuit to emit a maximum amount of energy and which may be
determined accurately only by means of a hot-wire ammeter placed in the
aerial circuit. The proper gap length is then indicated by the maximum
deflection of the meter.
The correct adjustment of the gap may be approximated by the experienced
operator. If too short, the spark will be hissing and flaming and is in
extreme cases red or yellow colored. It should be lengthened out until
it is thick and white and a slight increase in sound is noted. The spark
should not be, as commonly thought, stringy and crackling.
Arcing is often caused by insufficient condenser capacity in the
circuit. But if the capacity is increased to remedy the arcing, the gap
length should also be increased or otherwise the potential of the
condenser may not be sufficiently raised to permit a disruptive
discharge to take place.
[Illustration: Fig. 59. Spark Gaps.]
The best spark gap for use with a small untuned transmitter, making use
of neither tuning helix nor condenser, is shown at the left in Fig. 59.
It consists of two brass balls supported on suitable double binding
posts. The balls tend to thicken the spark and make it more disruptive
than if it passed between rods or points. The balls also increase the
potential required to leap the gap without increasing its length and
permit the aerial to become more highly charged before a discharge takes
place. For this reason the balls or knobs must always be kept free from
small rough spots or points by frequent polishing.
Silver has some peculiar property which makes it the most efficient
material for a spark gap, but its cost prohibits its use and so brass or
zinc, which are next in order, is generally used.
The type of gap illustrated at the right in Fig. 59 is very convenient
and efficient for small tuned transmitters employing coils of low power.
[Illustration: Fig. 60. Spark Gap.]
The zinc tips are pieces of 3/8-inch zinc battery rod 3/4 inch long.
They are bored and threaded to fit a pair of brass rods 1/8 inch
diameter and 2 1/2 inches long. The rods are supported by two double
binding posts and are fitted with two pieces of hard rubber rod 1/2 inch
diameter and 1 inch long to serve as handles. If desirable one of the
rods and binding posts may be threaded so that the adjustment of the gap
can be accomplished by revolving the handle and screwing the gap in or
out.
A gap of the size and type just described would not be suitable in
connection with a large transformer or an induction coil. The heavy
discharge of such powerful instruments would very rapidly heat the small
brass parts and corrode or even melt the zinc. The spark gap such as is
shown in Fig. 59 may be used with a very large coil or the two kilowatt
transformer if the same proportions are kept and it is made one-half
again as large. Using the dimensions shown in the figure, the spark gap
is suited to either the open core or the 1/4 and 1/2 kilowatt
transformers.
Two pieces of hard rubber rod, 3/4 inch diameter and 2 inches long serve
as the standards. Three grooves 1/8 inch wide, 1/8 inch deep and 1/8
inch apart better the appearance and reduce the liability of leakage but
are not necessary. Two holes 2 inches apart are bored in _S_, a strip of
brass 3 x 3/4 x 1/4 inches. Two screws pass through these holes into the
rubber standards _BB_ and hold _S_ firmly in position. The end of _S_ is
bent down at right angles and bored to receive a binding post. A hole is
bored in _S_ halfway between the standards and tapped to receive a
threaded brass rod 2 inches long and 1/4 inch diameter. The lower ends
of the hard rubber standards are each bored and tapped to receive a
screw which fastens them to the base. The electrodes are zinc or brass
cylinders 3/4 inch diameter and 5/8 inch long. The upper electrode is
adjustable by means of a knurled hard rubber head 2 inches diameter and
1/2 inch thick. The lower one is fastened to a brass plate 2 inches
long, 1 inch wide and 1/8 inch thick. One end of the plate is fitted
with a binding post. The base is a piece of hard rubber 5 1/2 x 3 inches
x 3/4 inch.