Spark Gap Efficiency and a New Type Gap 



By A. S. Blatterman, B. S. 



AS is the case with nearly all of the 

 apparatus used in wireless teleg- 

 raphy, the spark gap since the first early 

 experiments has undergone a great many 

 changes. In early days the spark gap 

 consisted simply of two brass balls. At 

 that time this arrangement answered the 

 purpose fairly well, though it was real- 

 ized that blackening and pitting of the 

 surface of the balls, due to the oxidizing 

 and corroding effect of the spark, were 

 detrimental. It was thought at first that 

 the only function of the gap was its ac- 

 tion as an automatic switch to suddenly 

 connect the two halves of a capacity cir- 

 cuit charged to opposite potentials, thus 

 suddenly relieving the existing condition 

 of electric strain so that oscillations were 

 produced. This simple property of the 

 gap is indeed a basic requirement, but 

 in modern practice is complementary to 

 another which must now be classed as 

 almost equal in importance for efficient 

 functioning of the apparatus. 



With the introduction of higher pow- 

 ers in transmitting it was soon found that 

 the stationary brass ball gap gave rise to 

 a power arc following the first discharge 

 of the condenser, which effectually pre- 

 vented further oscillations. This was 

 partially overcome by the use of trans- 

 verse magnetic fields applied at the spark 

 gap to blow out the arc, and with the 

 same purpose a blast of compressed air 

 was often introduced into the gap. Fur- 



FIG. 1 . 



Highly Damped Oscillations Produced by a Parallel 

 Plate Quenched Gap. 



ther experience led to the utilization of 

 resonance in the transformer circuits, 

 the inductance in the circuit preventing 

 the heavy rushes of current necessary 

 for maintaining the arc. 



The spark rate of apparatus using this 

 early spark gap in connection with in- 

 duction coils, and later with transform- 

 ers, was very irregular, so that the signal 

 produced in the telephones at receiving 

 stations was of a more or less crackling 

 or sometimes mushy sound, often diffi- 

 cult to separate from the similar sound 

 effects of atmospheric electricity. It was 

 also found by Wien, Austin and others 

 who investigated the subject experiment- 

 ally that the telephone receiver as well 

 as the human ear is not as sensitive to 

 low pitched sounds as it is to those of 

 higher frequency, in the neighborhood of 

 900 or so vibrations per second. Ac- 

 cordingly, methods began to be devised 

 for producing regular sparking rates of 

 higher frequency than had heretofore 

 been used. 



One method employs a disk carrying 

 a number of metallic studs evenly spaced 

 around its periphery and revolving at 

 high speed between stationary electrodes, 

 the whole device constituting an appara- 

 tus which gives a regular sparking rate 

 of high tone when used as a spark gap 

 in the condenser circuit. Another meth- 

 od employs an alternator of relatively 

 high periodicity, viz., 500 or more cycles 

 per second, with a special spark gap so 

 constructed and adjusted as to obtain 

 only one spark per alternation. At 500 

 cycles the ordinary spark gap falls far 

 short of permitting this condition, be- 

 cause the time of an alternation is so 

 brief that any arcing prevents the de- 

 sired clearing of the gap before the fol- 

 lowing alternation. Hence, individuali- 

 zation of the sparks is lost and the tonal 

 quality is impure and mushy. 



What is required is a rapid regularly 

 recurring discharge which is damped out 

 after a few oscillations. When a circuit 

 containing the usual form of spark gap 

 is coupled to a second circuit the oscil- 



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