652 PROCEEDINGS OF THE AMERICAN ACADEMY. 



There is here no trace of anomalous magnetization. 



If a strong current running through a solenoid containing a core of 

 magnetizable metal, be suddenly broken, there will be, under favorable 

 circumstances, an oscillatory discharge across the spark gap, and accord- 

 ing to Wiedemann's theory, which was based upon the numerous experi- 

 ments of early observers * upon the magnetization of steel needles by 

 discharges from Leyden jars, the phenomena of anomalous magnetiza- 

 tion are to be explained by the action of oscillating currents rapidly 

 decreasing in intensity, induced in the outer portions of the core under 

 test. Through the kindness of Mr. William Otis Sawtelle, who has a 

 large revolving mirror driven by a powerful motor, and devices which 

 he has himself designed and constructed' for photographing electric 

 sparks under various conditions, we were able to make sure that in the 

 cases of the apparatus which we used in our experiments upon anoma- 

 lous magnetization the discharge, when we suddenly opened the circuit, 

 was uniformly oscillatory in character. Mr. Sawtelle and Mr. Coulson 

 photographed a large number of these sparks ; and, from their results, 

 there cannot be any doubt, I think, that there were usually several hun- 

 dred reversals in direction while the visible discharge lasted. With one 

 of our solenoids, the period proved to be about l/58000th of a second, 

 and Professor G. W. Pierce, who most kindly tested one of our coils by 

 itself, showed that such frequencies were to be expected. In each of 

 the spark photographs, the record crossed the plate many times and 

 the growth of the spark length with the time when the circuit was sud- 

 denly opened could be studied from them. It appeared that the man- 

 ner of throwing the circuit open had very little effect upon the character 

 of the discharge. When the current in the solenoid circuit is brought 

 to zero by the continuous introduction of more resistance into the cir- 

 cuit, we do not expect that alternative currents will be induced in the 

 core. It is difficult, however, to get any satisfactory theory upon 

 which to base a mathematical investigation of the results of currents 

 induced in a core of soft iron by oscillations decreasing in amplitude in 

 a neighboring circuit. Even if the courses of such currents in a non- 

 magnetizable core could be satisfactorily treated, and this seems difficult 

 without a more accurate knowledge than we have about the behavior of 

 the exciting current oscillations, we should not have any clear light 



' Savary, Ann. de Chim. et de Physique, 34, 1826. Von Liphart, Pogg. 

 Ann., 116, 1862. Paalzow, Pogg. Ann., 117, 1862. Reiss, Pogg. Ann., 122, 

 1864. J. Henry, Scientific Writings, pp. 203 and 293. Rayleigh, Phil. Mag., 

 38, 39, 1870. Rutherford, Phih Trans., 189, 1897. Wilson, Electrician, 51, 

 1903. Fleming, Proc. Roy. Soc, 74, 1903. 



