958 



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 



