Induct ion- Coil Potentials. 583 



being connected in the primary circuit and the current kept 

 at 2*0 amperes, similar results were obtained. At 103 mm. 

 sparks passed freely with "05 mfd., none with *025. The 

 smaller capacity also failed at 98 mm. At this current also 

 •05 was superior to *1, and there was again a smaller 

 maximum at about '3 mfd. 



In the next experiments Lord Rayleigh's method of cutting 

 the primary circuit with a projectile was followed, the object 

 being to compare the two cases C 1 = *05 mfd., and C^O. 

 An air-gun was first used, which fires a pellet at about 

 500 feet per second. With one cell 19-mm. sparks were 

 obtained every time with the condenser, none without the 

 condenser at this distance or at 18 mm. 



Still following Lord Rayleigh''s plan * a service rifle was 

 next used. It was said to fire a bullet at 2000 feet per 

 second. Again with one cell, at 19 mm. there was sparking 

 every shot with the condenser, none with the condenser 

 removed. With the condenser sparks also passed at 2 cm. 

 Similar results were obtained with bullets reduced to about 

 one-half, sparks passing at 19'6 mm. with the condenser, 

 none without. At this distance *025 mfd. also failed. 



With two cells (i about 2*2 amperes) the sparking distance 

 was not so steady, and it was consequently more difficult to 

 fix upon a distance which enabled one to decide in favour of 

 the presence or absence of the condenser. The results were 

 therefore not so conclusive, but whenever a difference was 

 shown at consecutive shots it was always in favour of the 

 condenser. Thus, of three successive shots with the balls at 

 125 mm., the first and third being without, the second with 

 the condenser ('05 mfd.), the second alone gave a spark. The 

 longest spark obtained without the condenser was 120 mm. 



When an amperemeter and rheostat were connected in the 

 primary circuit, and the current was kept at 2*0 amperes, 

 the same condenser gave a spark at 111 mm., while at shots 

 immediately before and after no spark appeared without the 

 condenser. Again, at 107 mm. (reduced charge) no spark 

 passed without the condenser, while at two shots with the 

 condenser (before and after) a spark passed each time. In 

 other instances two consecutive shots at the same distance 

 showed a superiority of the condenser; in no such case did a 

 spark pass without the condenser when it failed with the 

 condenser f. 



* Phil. Mag. ii. p. 593 (1901). 



t I am indebted to my colleague Mr. A. H. Ferguson, and to 

 Mr. J. A. Owen, who joined me in the rifle experiments and verified 

 the observations. I also wish to thank Sergt.-Major MeCracken, of the 

 College O.jT.C, for his assistance in these experiments. 



