Page:Popular Science Monthly Volume 92.djvu/974

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��Popular Science Monthly

��"missed sparks," a clear chord-like tone is developed. This tone, when heard at a recei\'ing station, is much easier to read in the presence of strong atmospheric disturbances than is the low frequency- rattle produced by 60 cycle current and a fixed simple spark gap, such as shown last month, and the increase in signalling effectiveness thus gained more than compensates for the loss in conversion efficiency.

Operation of the Synchronous Gap

What would happen if the rotary gap were slowed down until only one spark could pass per half-cycle of condenser voltage? This will depend mainly upon two factors: first, the instant at which the minimum break-down potential of the gap occurs, with respect to the condenser voltage curve, and, second, the value of the minimum break-down potential for which the gap is adjusted. To get the best results, the gap should reach its break-down point just at the instant the condenser reaches its maximum charge, as shown in Fig. 43, Here the dashed line again represents break-down voltages, and it is seen how the gap reaches its favorable position for sparking just as the condenser secures its maximum charge. To maintain these conditions it is neces- sary to mount the rotary gap element directly upon the shaft of the generator which produces the alternating current for the power transformer, so that the time-relation between the two variables will be strict and unchanging. A careful adjustment must be made, by moving the fixed electrodes backward or for- ward around the circumference of the gap, so that the shortest gap length occurs just when the condenser is ready for discharge; otherwise no spark will pass, or else only part of the energy will be drawn from the condenser at each dis- charge. This method of working is called the synchronous discharge, since the applied voltage and the gap-discharge voltage vary automatically together or synchronously. It provides what is probably the best method of securing maximum power together with a spark regularity so perfect that a clear musical spark tone is had at any frequency. To get high pitched spark tones, how- ever, a fairly high frequency alternating

��current must be used, since there is only one spark for each half cycle. Thus a .500 cycle current, as shown in Fig. 43, will produce a spark tone of 1,000 im- pulses per second. As before, each dis- charge generates a group of radio fre- quency oscillations in the primary circuit consisting of the condenser, spark gap and inductance Li (of Fig. 41).

It should be noted that in general the non-synchronous method of operation involves the use of a rotary gap driven by a separate direct current motor with- out any particular relation to the input frequency, and that the sparking and missing times occur at random. In Fig. 42 is shown a perfectly adjusted relation

���Time in ihousandths of seconds- Fig. « Break-down point should be reached when the condenser reaches the maximum charge

between the gap frequency and that of the applied current which is almost im- possible to hold in practice, although it may often exist for short times. With the separately driven gap it is possible to slow down the discharge frequency until it is exactly twice that of the applied alternating current power, and so to ap- proximate the synchronous discharge condition. It is impossible to maintain the instant of discharge correctly in phase (or in step) with the power current in this way, however, and consequently for best synchronous operation, the direct mechanical connection of the rotary gap and the generator, must be relied upon.

Construction of the Quenched Gap

A fourth type of gap, shown in Fig. 44, is largely used in spark transmitters. This usually consists of highly cooled enclosed parallel sparking surfaces, often of silver, which are mounted in pairs and separated only about 1-100 in. The sparking potential of such a gap unit is about 1,000 volts, and to build up the breaking-down potential to a higher value

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