584 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



There seems to be in these cases neither a magnetic time lag nor any- 

 sensible von Waltenhofen effect. 



If an electromagnet has two exciting coils, and if one of them be 

 attached to the. terminals of a battery, the form of the battery current 



Figure 41. The current in one coil of the toroid DN which is in series 

 with an oscillograph and a battery, follows the curve ODZ or the curve OBP, 

 according as another coil wound on the same core is open or closed in itself. 

 When at the point D, the second coil which has been open is suddenly closed, 

 the oscillograph record gives the curve DBQ which except at the very begin- 

 ning can be exactly superposed upon the upper part of the line OBP. 



will depend upon whether the second coil is open or closed on itself, 

 and the difference is usually noticeable even when the magnet has a 



W 



TIME. 



Figure 42. 



large solid core in which eddy currents are being induced. Figure 39 

 shows curves taken under the two conditions just mentioned for both 

 the electromagnet TP and the toroid DN. To determine whether the 

 closing of the second coil in the case of the electromagnet where strong 



