Primary Capacity for Tesla Coils. 65 



circuits, the adjustment being made by varying the primary 

 capacity. 



A number of methods were tried for ascertaining when the 

 potential in the secondary was a maximum. One of these 

 consisted in sparking from a secondary terminal to an insu- 

 lated electrode. The distances at which the sparks just 

 failed to pass to the electrode were taken as indicating the 

 maximum terminal potential for each adjustment of the 

 capacity in the primary circuit. This method, though giving 

 comparative values of the terminal potential, did not appear 

 sensitive enough for the determination of an optimum 

 capacity*. 



The method finally adopted consisted in measuring the 

 distance from the terminal at which a neon tube just failed 

 to glow when held horizontally opposite the upper terminal 

 of the coil, the end of the tube remote from the terminal 

 being well earthed. This method, although the most sensitive 

 of those tried, was not capable of great exactness; and as 

 the potential of the secondary varied slowly in the region of 

 the maximum, the adjustment of the primary capacity to its 

 most effective value was a difficult matter, and could not be 

 made very accurately. The method, however, is one in 

 which practically no disturbance of the conditions is intro- 

 duced, owing to the fact that the tube glows when at a 

 considerable distance from the terminal of the coil. 



To ensure that the distance of the tube from the coil 

 depended upon the terminal potential alone and not upon 

 the length of the coil, a wire about 2 metres long was 

 attached to the upper secondary terminal. This wire pro- 

 jected horizontally from the coil and was suspended by silk 

 cords from the ceiling. The neon tube was held opposite 

 this wire at right angles to its length. Distances were 

 always measured when the tube was in this position. The 

 glow of the tube was affected by the presence of earthed 

 bodies. It was important tnat the disposition of these bodies 

 near the wire should remain constant during the experiments. 

 This condition was secured by the projecting wire being at 

 a fixed height from the floor, all other conducting bodies 

 (including that of the experimenter) being kept in a fixed 

 position when readings were being taken. 



* It may be mentioned that an aluminium-leaf electroscope connected 

 to one of the secondary terminals also shows a distinct maximum as 

 the primary capacity is varied, su'.m that if the capacity is increased or 

 diminished from this value, the deflexion of the leaf falls. There is thus 

 an optimum capacity for " mean square," as well as for " maximum " 

 secondary potential. 



Phil. Mag. S. 6. Vol. 31. No. 181. Jan. 1916. F 



