Induction-Coil Potentials Page 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 \(C_1 = 0\). 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 (\(i_0\) 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^†.

^* Phil. Mag. ii. p. 593 (1901).
^† 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 McCracken, of the College O.T.C., for his assistance in these experiments.