OF ARTS AND SCIENCES. 115 



One of these curves is shown at 1, Figure 1, the core in this case 

 being a cylindrical bar of Norway iron 4i inches long and | of an inch 

 in diameter, and the diaphragm an ordinary disk of ferrotype iron 2j^g- 

 inches in diameter and ^J^ of an inch thick (No. 31 B. W. G.). 



Table I. gives the data from which Figure 1 was constructed. The 

 strength of field is given in terms of the tangents of the angles of de- 

 flection of the magnetometer needle. The induced current is given in 

 arbitrary units, as only relative values are needed. A determination 

 of the value of the deflections was made by observing the excui-sion 

 due to the discharge of a condenser through the ballistic galvanometer, 

 and it was found that the abscissa 100 on the curves corresponds to a 

 sudden discharge of approximately 0.00000097 of a coulomb through 

 the coils of the galvanometer. 



Cores of Bessemer steel and of untempered soft steel were also used, 

 with results given in Tables II. and III. 



