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]
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 little 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.