OP ARTS AND SCIENCES. 



239 



the mafrnetic change which a small current in the line coil would 

 produce. 



The current which traversed the line coil was six milliamperes, a 

 value which was used in most of the measurements, it having been 

 found by trial that the general character of the results was the same 

 as when a smaller line current was used, while the greater magnitude 

 of the deflection of the current induced in the secondary measuring 



/To an oo *o iso 



Fig. 3. 



circuit allowed more satisfactory readings to be made than if the line 

 current were smaller. 



A corresponding series of measurements with a diaphragm of ferro- 

 type iron, No. 31, B. W. G., 0.010 in. in thickness, gave the results 

 found in Table VIII. and shown in Curve 31, Figure 2. Curve 2, 

 Figure 2, is constructed with abscissas equal to the difference of the 

 corresponding abscissas of Curves 31 and 1, and shows for various 

 strengths of field the increase of the induced current due to the pres- 

 ence of the diaphragm, an interesting illustration of the action to 

 which Du Moncel ascribed the chief efficacy of the diaphragm. 



