538 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



interval the length of which depended upon the final value of the coil 

 current, the electromotive force in its circuit, and the magnetic condi- 

 tion of the core at the start ; then a temporary current which had a 

 sensible value for perhaps ten or fifteen seconds passed through the 

 secondary, though if a sensitive ballistic galvanometer instead of the oscil- 

 lograph had been connected with the coil it would have been made clear 

 that the temporary current had not wholly disappeared at the end of this 

 interval. When, after the current in the main circuit had been apparently 

 steady for a minute or two the coil circuit was suddenly broken, the cur- 

 rent in the secondary became almost immediately evident, soon attained 



Figure 22. 



its maximum value, and then died slowly away so that the mirror seemed 

 to reach its zero again after about thirty seconds. 



Figure 21 shows a typical oscillograph record accurately reproduced. It 

 was obtained with the air gap closed and with a current of 3.12 amperes 

 in the main circuit from a storage battery of about 84 volts. For five or 

 six seconds after the start there was no sensible indication in the sec- 

 ondary coil, though by that time the main current (1) had attained about 

 three fourths of its final value. The greatest value of the current in the 

 secondary did not occur until rather more than fourteen seconds after the 

 current in the primary had begun, and the secondary current had appar- 

 ently died out in less than thirty seconds from the beginning. The ordi- 

 nates of the curve of the current in the main circuit represent amperes ; 



