1912] Hiijh Frequency Currents. 491 



WEEKLY EVENING MEETINCJ, 



Friday, May 17, 1912. 



Sir William Crookes, O.M. LL.D. D.Sc. F.R.S., Honorary 

 Sticretary and Vice-President, in the Chair. 



W. DuDDELL, Esq., F.R.S. M.R.I. 



High Frequency Gurrmts. 



The subject of High Frequency Currents is an extremely fascinating 

 'one, for not only are the effects obtained very beautiful, but also 

 there is an air of mystery about them which lends to their attractive- 

 ness. This mystery is in a large measure due to the fact that the 

 ordinary laws governing the flow of electric currents seem to be 

 violated, though on closer inspection this is not really the case. 



The subject is now a very large one, and I propose to-night 

 merely to pass in review the different methods of generating high 

 frequency currents and to illustrate their principles experimentally. 



Before proceeding it will be well that I should explain what I 

 mean by high frequency currents, and for this purpose I shall have to 

 rapidly survey a certain amount of elementary matter. 



If an electric current flows continuously in the same direction 

 through a wire or other circuit, it is said to be a continuous or direct 

 current. If, however, the current flows along the wire first in one 

 direction, then in the other, alternately reversing its direction of flow, 

 it is said to be an alternating current. When the growth and decay 

 of the current and its reversal take place in a regular manner, we 

 speak of the number of times per second that the current goes 

 through a complete cycle of change as its frequency. 



This is perhaps best illustrated by considering the motion of a 

 clock pendulum. The little pendulum which I am using to write 

 down its motion on the smoked glass in the lantern has a frequency 

 of two. 



In the case of most alternating currents used for electric lighting 

 the frequency ranges between 50 and 100, and in the case of the 

 current supplied to this lecture theatre from Deptford the frequency 

 is 85. These might be thought to be high frequencies, but they are 

 really not at all high. The frequency of the current in the telephone 

 wires — say, for instance, when the electrophone is transmitting a high 

 musical note — might be as high as 1000. The upper limit at which 

 sounds appear to have a clearly musical character is probably of the 

 order of 10,000 per second, and the limit of audibility some 20,000 

 per second, depending on the individual. 



