108 



IOWA ACADEMY OF SCIENCE 



Pig. I. 



Use of Ballistic Galvanometer. 



pendulum reached key k3 the galvanometer was thrown in circuit, and 

 when key k4 was reached the galvanometer was thrown out of circuit. The 

 distance between keys kl and k2 gave the interval of time ^t. After 

 the pendulum had thrown k2 k was opened by hand. The following 

 table gives the observations that were made with the selenium cell: 



