Page:Popular Science Monthly Volume 92.djvu/813

 Wireless Work in Wartime

X. — The Non- Synchronous Gap Radio Transmitter By John V. L. llogaii

��HAVING taken up, in the previous article, the general operation of the spark gap and primary oscillation circuit of a typical radio sending station, further and more specific types of spark gap may now be considered. In the il- lustration Fig. 38, printed last month, was shown a simple fixed air-cooled gap; and

���Tig. 59

An old type of a rotary spark gap which has been in general use for some time for wireless

the accompanying Fig. 39 shows a type of rotary spark gap which has had wide use. This consists of a rotating conducting arm A, having spark electrodes on both ends and mounted upon a driving shaft so that it may spin between two semi-cir- cular frames B, B. Supported on the frames are a number (in this instance ten) of fixed, equally-spaced electrodes. The five on one frame are connected together and act as one terminal of the spark gap; the other five form the other terminal, being connected together in the same way. The length of the studs and the separation between them is so chosen that when the rotating arm is almost directly in line with any one pair there will be only two short spark gaps in the circuit. If at this time the transformer (and condenser) voltage is near its maximum, sparks will pass and the condenser will discharge with oscillations, as previously explained. If the rotating arm has passed out from a position almost directly between two stationary studs, however, not even the maximum secondary potential of the transformer can force a spark to jump.

��The Disk Rotary Gap

Another type of rotary spark gap is shown in Fig. 40. Here there are two stationary terminals D and E, and be- tween these there rotates a spoked wheel F. Each spoke has a spark gap tip G at its outer end; the length of the spokes and their distance apart is selected so that sparks can pass only when they are almost directly in line with the stationary elec- trodes. It is evident that these two types of rotary spark gap have much in com- mon; in both of them the gap length is continually changing, and in both the gap is cooled and kept clear of conducting gases by the air circulation stirred up by the rotating member.

There are two general ways of using the rotary spark gaps. The gap of Fig. 39 is generally used according to one of these, the "non-synchronous" method, and has consequently come to be known as the non-synchronous rotary gap. The second type, of Fig. 40, has had its widest use according to the second or "synchronous" method, and is therefore often called the synchronous rotary gap. Neither of these names is strictly correct, however,

���Fig. 40

The second type or synchronous method is usually called the synchronous spark gap

since there is no reason why either gap should not be used according to either the synchronous or non-synchronous method. This will appear from an ex- amination of the two types of operation.

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