cal circuit. The
difference in time made by the two relays, although very slight, will be
found to affect very considerably the quality of the received pictures.
Renewing the platinum contacts is also a great expense, as they are soon
burnt out where a heavy current is passed. If the distance experimented
over is short so that the power required to operate the spark-coil is not
very heavy, one relay will be sufficient providing the contacts are massive
enough to carry the current safely. It is useless to expect any of the
ordinary relays in general use to work satisfactorily at such a high speed,
and in order to compensate for this we must either increase the time of
transmitting, or, as already suggested, make use of a coarser line screen
in preparing the photographs.

For reasons already explained, all points of make and break should be
shunted by a condenser. The effective working speed of an ordinary type of
relay may be anything from 1000 to 2500 dots a minute, depending upon
accuracy of design and construction.

In the wireless transmission of photographs it {27} is absolutely essential
to use some form of rotary spark-gap, as where sparks are passed in rapid
succession the ordinary type of gap is worse than useless. When a spark
passes between the electrodes of an ordinary spark-gap, Fig. 12, we find
that for a fraction of a second after the first spark has passed, the
normally high resistance of the gap has been lowered to less than one ohm.
If the column of hot gas which constitutes the spark is not instantly
dispersed, but remains between the electrodes, it will provide an easy path
for any further discharges, and if sparks are passed at all rapidly, what
was at first a disruptive and oscillatory discharge will degenerate into a
hot, non-oscillatory arc.[4]

[Illustration: FIG. 12.]

Two forms of rotating spark-gaps are shown in Figs. 13 and 14, and are
known as "synchronous" and "non-synchronous" gaps respectively. In the
synchronous gap the cog-wheel is mounted on the shaft of the alternator,
and a cog comes opposite the fixed electrode when the maximum of potential
is reached in the condenser, thus ensuring a discharge at every alternation
of current. With this type of gap a spark of pure tone is obtained which
{28} [Illustration] [Illustration] is of great value where the signals are
received by means of a telephone, but where the signals are to be
mechanically recorded the tone of the spark is of littl