492 



3fr. W. Dnddell 



[May 17, 



It is possible to produce electric currents having frequencies 

 ranging from 10,000 per second up to many millions, and it is 

 currents between these frequencies that I call high frequency currents. 

 This evening I am going to devote my attention more especially to 

 frequencies in the range between ten thousand and a million, as 

 currents of these frequencies have a large number of practical 

 applications, such as wireless telegraphy, high frequency treatment 

 of diseases, electrical cautery, thermo-penetration, etc. 



The methods of producing high frequency currents may be divided 

 broadly into three classes, namely : (1) Alternator methods : (2) 

 methods based on the oscillatory discharge of a Leyden jar: (3) arc 

 methods. Although the alternator method of producing very high 

 frequencies has not had the same general application as the Leyden 

 jar discharge, I will treat it first for two reasons : Firstly, it is some- 





Fig. 1. — Alliance Alteenatoe. 



what easier to understand ; secondly, there seems a probability that 

 the high frequency alternator will l)e very greatly improved and 

 developed, and soon come into more general application. 



Alternators were among the first machines invented for producing 

 electric currents on the well-known Faraday principle of magnetic 

 induction. Among the early machines which will illustrate the 

 principle I may mention the Alhance machine, which was largely 

 used in lighthouses, from I860 onwards. The essential parts were a 

 number of magnets, and wires on bobbins which passed across the 

 poles, in which electric currents were induced (Fig. 1). The frequency 

 of these machines was about 50 per second. 



Consider a single wire passing a north pole. A current will be 

 induced in it in a certain direction, according to the well-known 

 principles discovered by Faraday. When the v^ire passes across the 



