568 



PROCEEDINGS OF THE AJMERICAN ACADEMY. 



that the metal be made to go over the same magnetic journeys which 

 it will later be required to make, and at the same speed. 



Before we discuss this anomalous magnetization more carefully, we 

 may stop for some moments to study the records of an oscillograph in 

 circuits which contained either the electromagnet TP, Figure 28, which 

 has a solid core, or a certain toroid (DN), about 41 centimeters in 

 mean diameter, the core of which was made of about 25 kilograms of 

 fine, soft, varnished iron wire. It will be seen from Figures 29, 30, 31, 

 32, 33, 34 that the phenomena are in general what we should expect 



Figure 29. The curve OHS shows the manner of growth of a current in 

 the coil of the magnet TP when the poles are separated by about three 

 inches. WK shows the rise of the same current when the poles are nearly- 

 closed by the insertion of a planed block of iron between them. HLT shows 

 the effect of suddenly dropping the block in while the current is growing. 



to find in similar circuits without iron, though eddy currents and the 

 time taken to make the mechanical changes modify somewhat the 

 courses of the currents in the exciting circuits. 



The Anomalous Magnetization of IroNc 



In 1863 von Waltenhofen first called attention to the fact that if 

 an increasing current (C) ending in the maximum value {€') be sent 

 through a long solenoid, the final value of the magnetic moment of a 

 bar of soft iron in the solenoid, which was at the outset demagnetized, 

 will depend not only upon the final strength of the current, but also 

 upon the manner of growth of C in attaining this intensity. This 

 moment will be greater if the current be suddenly applied in full 

 strength than if it be made to grow slowly, either continuously or by 



