114 



PROCEEDINGS OP THE AMERICAN ACADEMY 



carrying a cam moved by a weight, a rapid inward push of definite 

 amount was given to the diaphragm, thereby inducing a current in the 

 coil already referred to, and so deflecting the needle of the ballistic 

 galvanometer. The soft iron bar was also surrounded by a second 

 helix, through which was passed a current from a storage battery, 

 serving to magnetize the core. A tangent galvanometer inserted in 

 this circuit gave the strength of the magnetizing current. A mag- 

 netometer placed in the prolongation of the axis of the core, which last 

 occupied an east and west position, made known the relative strengths 

 of the field produced by the core under different conditions of magnet- 

 ization. 



Corresponding observations of the magnetometer reading, and of 

 the current induced when the diaphragm was moved by the cam, were 

 made throughout a widely varying range of strength of field, and the 

 results were represented graphically by constructing a series of curves 

 in which ordinates represent the relative strength of field, and abscis- 

 sas the current due to a given predetermined throw of the diaphragm 

 (about T ^ of an inch), as ascertained from the readings of the ballistic 

 galvanometer. 



o 



s 



FERROTYPE IRON. 



K «> i» 60 70 



INDUCED CUfiRCHT. 



Fig. 1. 



