314 PROCEEDINGS OF THE AMERICAN ACADEMY. 



say one hundred and fifty thousand ampere turns about this core in 

 less than about 30 seconds under any practicable voltage. Of course 

 the process is not completed even in this time, but the amount of elec- 

 tricity carried by the induced current after 30 seconds can be made 

 relatively very small. Indeed for the shape of current practically en- 

 countered with this apparatus, the duration of the flow might be 60 

 seconds without causing a decrease of more than a fraction of one per 

 cent in the throw of the galvanometer W. 



I wish to express my obligation to the Trustees of the Bacbe Fund 

 of the National Academy of Sciences for the loan of apparatus used in 

 studying for this paper some of the induction current diagrams. 



The Jefferson Laboratory, 

 Cambridge, Mass. 



