e consequence. In a
non-synchronous gap a separate motor is used for driving the toothed wheel,
and can either be mounted on the motor shaft or driven by means of a band,
there being no regard given to synchronism with the alternator. The fixed
electrode is best made long enough to cover about two of the teeth, as this
ensures regular sparking and a uniform sparking distance; the {29} spark
length is double the length of the spark-gap. The toothed wheel should
revolve at a high speed, anything from 5000 to 8000 revolutions per minute,
or even more being required. The shaft of the toothed wheel is preferably
mounted in ball-bearings.

Owing to the large number of sparks that are required per minute in order
to transmit a photograph at even an ordinary speed, it is necessary that
the contact breaker be capable of working at a very high speed indeed. The
best break to use is what is known as a "mercury jet" interrupter, the
frequency of the interruptions being in some cases as high as 70,000 per
second. No description of these breaks will be given, as the working of
them is generally well understood.

In some cases an alternator is used in place of the battery B, Fig. 4, and
when this is done the break M can be dispensed with. In larger stations the
coil H is replaced with a special transformer.

The writer has designed an improved relay which will respond to currents
lasting only 1/100th part of a second, and capable of dealing with rather
large currents in the local circuit.[5] This relay has not yet been tried,
but if it is successful the two relays R and R' can be dispensed with, and
the result will be more accurate and effective transmission.

{30}

[Illustration: FIG. 15.]

The connections for a complete experimental station, transmitting and
receiving apparatus combined, are given in Fig. 15. The terminals W, W are
for connecting to the photo-telegraphic receiving apparatus Q, being a
double pole two-way switch for throwing either the transmitting or
receiving apparatus in circuit. There is another system of transmitting
devised by Professor Korn, which employs an entirely different method from
the foregoing. By using the apparatus just described, the waves generated
are what are known as "damped waves," and by using these damped waves,
tuning, which is so essential to good commercial working, can be made to
reach a fairly high degree of efficiency. {31}

The question of damped _versus_ undamped waves is a so