1912] 



0)1 Hifih Frpqiimcy Currents. 



511 



In using a genenitor of this sort for wireless telegraphy, messages 

 have been transmitted and recorded from a distance of about loOO 

 miles at tlie speed of over 100 words per minute. 



A large number of modifications of the arc method have been 

 produced, practically all of which consist in changing the nature of 

 the electrodes or the surroundings of the arc, so as to try and make 

 it still more unstable at high frequencies and large currents. Most 

 arcs seem to lose their instability after the current has passed a certain 

 limit. For this reason there is generally a best working current for 

 any arc. If it is required to deal. with more power than one arc can 

 transform, it is better to put the arcs in series. 



A trick which is very useful and which enables two arcs to be 

 burnt in parallel as far as the mains are concerned, and to be in series 

 as far as the oscillatory circuit is concerned, is illustrated in Fig. 17. 

 This is at times very convenient, as sufficiently high voltage direct 

 current generators are not always available. 



Fig. 17. — Connections for Arcs in Series Parallel. 



A very interesting generator of high frequency currents is that 

 invented "by Mr. S. G. Brown. It is difficult to say whether it is an 

 arc or a spark or what. The unstable conductor which takes the 

 place of the arc consists of an aluminium wheel, on the edge of which 

 a copper l;)lock rests (Fig. 18). The wheel is kept rotating, and the 

 contact is shunted by a condenser and self-induction. This arrange- 

 ment produces higli frequency currents quite easily, but I have never 

 been able to make it deal with any large powers. 



In conclusion, it is of interest to review in general terms the present 

 position of the three main methods of producing high frequency 

 currents. The most important application at the moment for high 

 frequency currents is wireless telegraphy. For this pm-pose frequencies 

 in the range between 30,000 and 1,000,000 are required. A few 



