" Skin "-effect in Electrical Oscillators. 5 



it was thinner than this " skin " ; and if so, what was the criti- 

 cal thickness in any particular case. 



In the experiments to be described oscillators were used 

 with frequencies approximately 375, 825, 2000, 3200 millions 

 per second, respectively. Substituting these values for n in 

 the above value (11) for x ; and taking /k=1, cr=1600, approxi- 

 mately, for copper or gold, 13,500 for platinum and 4,770,000 

 for electric-light carbon, we obtain the following table : 







Table I. 





'pniiPnPF 





■ - - Tin' pirn nw nf u ^lrin " 





n 



Copper. 



Platinum. 



Carbon. 



lllions. 



cms. 



cms. 



cms. 



375 



0*00033 



0-00092 



0-01749 



825 



0-00022 







2000 



0-00014 







3200 



o-ooon 







We can obtain an approximate value for the thickness of 

 the " skin " in another way. Suppose the conductor to be a 

 circular cylinder. From Stefan's formula (2) 



and assuming that, as with steady currents, the resistance is 

 inversely proportional to the area of the section used by the 

 current, the oscillatory current must occupy the R/B/th part of 

 the section. Since this portion is a thin layer next the surface 

 we have 



2irax = =r-, X tt« 2 , 



. /nil . 



= tray — X va j 

 a 



and x = — y — 



which turns out to be precisely the value we obtained before 



(ii). 



Hertz- stated that, as far as he could observe, the nature of 

 the metal out of which his resonator was formed had no influ- 

 ence upon the phenomena, but experiments by Bjerknesf did 

 not confirm this conclusion. He found the efficiency of the 

 metals : copper, brass, silver, platinum, nickel, iron, to be in 

 the order in which they are here named. By depositing elec- 



* Electric Waves, p. 45. 



f Wied. Ann., xlviii, p. 592, 1893. 



