1912] on High Frequency Currents. 501 



to beiug a comparatively good conductor, and it remains conducting 

 in the ordinary way until the current finally ceases. I shall have 

 some more to say later on the subject of the change of conductivity 

 of the spark gap. Returning to our circuit containing our condenser, 

 spark gap and self-induction, if the condenser be charged to a suffi- 

 ciently high voltage the spark gap Avill break do^Yn, a spark will take 

 place, and during the time that the gap is conducting oscillations will 

 take place. In order, therefore, to produce the repeated sets of dis- 

 charges of the condenser, all we have to do is to repeatedly charge 

 the condenser to a sufficiently high voltage, and this is easily accom- 

 plished by charging it from a transformer connected to an ordinary 

 low frequency alternator. For instance, with the ordinary 85 fre- 

 quency current available I can obtain one discharge each time the 

 condenser is charged either positively or negatively — that is to say, 

 170 discharges per second, each of which discharges consists of a 

 series of high frequency oscillations. 



This is the usual method of producing high frequency currents, 

 and is the method used by Tesla for his most Ijrilliant experiments, 

 shown in this lecture theatre twenty years ago. It will be evident, 

 however, that the high frequency currents obtained by this means 

 differ fundamentally from those detained from an alternator ; for in- 

 stance, with an oscillation frequency of 1,000,000 per second, such as I 

 am using, and 50 oscillations for each discharge of the jar, the whole 

 discharge will last ^ohws of a second. As we are only obtaining 170 

 discharges per second, there is something like a 2()0th of a second 

 between the end of one discharge and the beginning of the next. 

 Avhich is a comparatively long time, about 100 times as long as the 

 time the oscillations last. Inventors have been struggling, and with 

 some success, to close up these gaps. 



It is obvious that if the discharges can be caused to take place 

 with greater rapidity by using a higher frequency alternator to charge 

 the jars, the gaps will be closed up, and this is the method originally 

 used by Tesla, and which is now being used in many of the wireless 

 transmitters, though in this case there is another reason for using a 

 greater number of discharges per second. For instance, the Marconi 

 Company, in their musical spark transmitters, use a frequency of 

 some 300 — that is to say, some 600 discharges per second — and' the 

 Telefunken Company as many as 1,000 discharges per second. 



Mr. Marconi has described in this lecture theatre the method he 

 uses in his trans-Atlantic stations. In this case, instead of charging 

 the Leyden jar or condenser from an alternator, it is charged from 

 a high voltage battery, and the sequence of discharges is produced 

 by means of a special form of spark discharger, which consists of a 

 disc fitted with studs round its edge. When this disc is rotated the 

 studs pass between the balls of the spark gap, and, corresponding to 

 the passage of each stud, a discharge takes place. If the disc is 

 driven at a very high speed a large number of discharges per second 



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