64 Mr. W. Morris Jones on the most Effective 



by a motor, and the discharge terminals o£ the coil were 

 made of zinc. 



To diminish damping due to brush discharge from the 

 terminals of the secondary of the Tesla coil, and to prevent 

 sparking between secondary and primary, the spark-length 

 used was only 1 mm., and this was kept constant throughout 

 the whole series of experiments. Brush discharge was not 

 altogether prevented even by the use of such a short spark- 

 length. 



A few preliminary experiments were made to ascertain 

 whether the primary discharge potential was independent of 

 the primary capacity and remained constant provided the 

 spark- length was constant. For this purpose two condensers 

 of widely different capacities were respectively put in series 

 with the spark-gap. Measurements of the discharge potential 

 were taken by means of an electrostatic oscillograph*. From 

 the results obtained for the two capacities used, the dis- 

 charge potential appeared to be independent of the primary 

 capacity. 



Measurement of Frequencies. 



All frequencies were measured by a wavemeter. These 

 were usually too high to be registered by the wavemeters in 

 common use in wireless telegraphy, and one suitable for the 

 purposes of the experiments was constructed as follows. A 

 standardized variable condenser was connected in series with 

 a single circular turn (58*16 cm. diameter) of copper wire 

 (*6376 cm. diameter) the self-inductance of which could be 



(QT5 \ 



log* 2 J. By 



means of a suitable switch two other coils of two and three 

 turns respectively could be substituted either separately or 

 in series for the single-turn coil. The range of the wave- 

 meter was thus widened, and all the frequencies required to 

 be measured came within this range. The extra coils were 

 standardized by comparison with the single-turn coil. A 

 carborundum detector with telephone connected across the 

 terminals of the condenser was used in all the experiments. 

 The frequencies of the oscillations in these experiments were 

 within the range -710 x 10 6 to 3'28 xJO 6 . 



Measurement of Maximum Potential. 



In these experiments comparative measurements were 

 required of the maximum secondary potential that could be 

 developed for various degrees of coupling of the Tesla coil 

 * E. T. Jones, Phil. Mag. xiv. p. 238 (1907). 



