Primary Capacity for Tesla Coils. 67 



adjustment was made when the primary circuit of the Tesla 

 coil was open and also when it was closed by a short wire 

 of: sufficient length to allow for the self-inductance of the 

 leads used when the primary was connected up in series with 

 the condenser and spark-gap. The resonance capacity in 

 the dummy circuit was noted in each case. The ratio of 

 the periods when these capacities were respectively in the 

 circuit were obtained from the wavemeter readings. If T 2 

 and T x 2 , to which the wavemeter readings are directly pro- 

 portional, represent the periods of the secondary when the 

 primary was respectively open and closed, then T 1 2 /T e 2 = l — k 2 

 from which P can be immediately obtained. The value of 

 P was also verified in some instances by determining the 

 minimum ratio of the oscillation-frequencies of the Tesla coil 

 system. 



The reading of the wavemeter when the " dummy " circuit 

 had been tuned to resonance with the secondary of the Tesla 

 coil, the primary being open, gave the period of the secondary, 

 and therefore the value of L 2 C 2 . The capacity in the 

 " dummy " circuit for this case was the " resonance " capacity 

 -satisfying the condition L 1 C 1 = L 2 C 2 . The " optimum " 

 primary capacity was found to be always greater than the 

 resonance value. 



The above completed the set of determinations required 

 for one value of P. The coupling was next increased a little 

 by removing turns from both ends of the secondary, and the 

 primary capacity again adjusted to the " optimum." The 

 whole set of observations was then repeated. These adjust- 

 ments and observations were made both with the primary 

 ooil at the middle of the secondary and the secondary 

 terminals insulated, and with the primary at the lower end 

 of the secondary and the lower terminal of the latter well 

 earthed. 



In the earlier part of the series of experiments all the five 

 turns of the primary coil were used ; but when about 100 

 turns had been removed from the secondary, it was found 

 that the " resonance " value of Ci became inconveniently 

 small for the range of the oil condenser. Consequently, in 

 the latter part of the series only four turns of the primary 

 coil were used. This reduces L^ and proportionately increases 

 the "resonance" value of C l5 thus making it more suitable 

 for the range of the oil condenser employed. 



The removal of a turn from the primary has, however, 

 another effect. It diminishes L 2 i and L ]5 but L x is reduced 

 in a greater proportion than L 2 i. Thus the ratio L 21 /Li is 

 increased; and according to the expression (1) the maximum 



¥2 



