The Laic of the Condenser in the Induct ion- Coil, 411 



lead screens. This penetrating radiation mar have its origin 

 in the radioactive matter which is distributed throughout the 

 earth and atmosphere. It was not found possible to obtain 

 sufficient excited activitv on a wire chartred neo^ativelv in 

 the open air to show the presence of a very penetrating 

 radiation due to it. The effect observed is too large to be 

 accounted for by the excited acti's'ity distributed on the walls 

 of the laboratory. 



(2) That all the substances examined give forth a radiation 

 of a not very penetrating character; that this is probably 

 the cause of all the residual ionization in the electroscope 

 when surrounded by heavy metal screens ; and that this 

 activity varies with different substances, being very low in 

 the case of brass. 



(3) The reduction by the experimental arrangements of 

 the number of ions produced per c.c. per second in air under 

 atmospheric pressure from 14 to 5. 



In conclusion, I wish to thank Prof. Rutherford, at whose 

 suggestion and under whose very kind supervision this 

 research was undertaken and carried out. 



McGm UniYersitT, June 23. 1903. 



LIV. On the Law of the Condenser in the Induction- Coil. 

 By James E. Iyes, JUi.L)., Instructor in Physics in the 

 University of Cincinnati^, 



[Plate XVII.] 



ri'^HE optimum capacity, in an induction-coil, is defined as 

 JL that capacity of condenser which, when placed around 

 the break, wiU give the longest spark in the secondary. It 

 has also been found by experiment to be the least capacity 

 that causes the sparking at the break to disappear, or if not 

 entirely to disappear to become very small. 



In the July number of the Physical Review I proposed a 

 formtda for it, based upon theoretical considerations, viz. : — 



av 



where K is the optimum capacity : T,, the steady current in 

 the primary at the instant of breaking the circuit ; i*,, the 

 velocity with which the circuit is broken ; and a some con- 

 stant which depends upon the medium in which the break is 

 made. Unfortunately for this theory, experiments made 



* Communicated bv the Author. 



