Primary Capacity for Tesla Coils. 63 



resistances are neglected, as given by the expression 



V '= L lV /(T^°#^4F^ (cos 2 ™^- cos 2 ™^*' W 



it is shown in the paper referred to that if k 2 = 0*265 the 

 optimum primary capacity is 2*128 times the "resonance" 

 value; and that if /c 2 = 36 (a value specially recommended 

 by Drude along with the " resonance" condition) the optimum 

 capacity is at least twice as great as the resonance value. 



It was suggested to me by Professor Taylor Jones that I 

 might test the conclusions by experiment, and also determine 

 •experimentally the optimum primary capacity and relative 

 values of the maximum secondary potential for various values 

 of P, with a view to obtaining data for a complete test of the 

 theory. 



In the experiments about to be described the coupling 

 •coefficient was varied by removing turns from the secondary 

 of the Tesla coil. In this process the primary self-inductance 

 Li remains constant, and the variation of L 2 i can be easily 

 determined and allowed for. The value of k 2 and the 

 optimum value of m were determined at each stage. The 

 same spark-length was maintained throughout the experi- 

 ments, so that V could also be regarded as constant. 



The Apparatus. 



At the beginning of the series of experiments the secondary 

 •of the Tesla coil consisted of 192 turns of bare wire (No. 16 

 S.W.G.) wound on a wooden octagonal frame smeared with 

 paraffin- wax, the immediate supports of the coil being covered 

 with indiarubber tape. Initially the coil was 80 cm. long 

 and the turns were 25 cm. in diameter. The primary coil was 

 of 5 turns of bare wire (No. 12 S.W.G.). The turns, about 

 30 cm. diameter, were 1 cm. apart and wound on an ebonite 

 frame of the same shape as that supporting the secondary. 

 The primary coil could be removed altogether from, or put 

 into any position over, the secondary, which was always used 

 with its axis vertical. The primary circuit of the Tesla coil 

 •contained the primary coil in series with an oil condenser 

 of variable capacity charged by an induction-coil and dis- 

 charged across a spark-gap. The primary current of the 

 induction-coil was interrupted by a mercury break driven 



* la this expression V is the induction-coil discharge potential, L 21 is 

 the induction coefficient of the primary on the secondary, k is the 

 coefficient of coupling, m is the ratio L 2 C 2 /L 1 C 1 , and n 1} n 2 ate the 

 frequencies of oscillation of the system. 



