PEIRCE. — BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 149 



given in this paper show clearly, from a current curve taken in the 

 exciting coil of a magnet which has a large solid core. 



It will be evident from what precedes that it is possible to predict 

 accurately the building-up curve of a current in the coil of an electro- 

 magnet with fine wire core, from 

 a corresponding hysteresis dia- 

 gram obtained by aid of a ballis- 

 tic galvanometer of long period, 

 and one of the old methods of 

 procedure. 



Figure 43 shows two reverse 

 current curves for a toroidal 

 magnet of about one third of a 

 henry inductance belonging to 

 the American Telephone and 

 Telegraph Company. The final 

 strength of the current was the 



same (1.42 amperes) in both cases, but the applied electromotive 

 force was 10.9 for the left-hand curve and 21.5 for the other. The 

 disturbing effects of eddy currents were here (as will be shown in the 

 sequel) wholly inappreciable. We should be justified in expecting 

 that each of these current curves would yield by aid of a good plani- 

 meter a hysteresis diagram substantially the same as any ballistic 

 step-by-step method would furnish for the same magnetic journey of 

 the core. 



Figure 43. 



SECONDS. 



The Influence of Eddy Currents upon the Apparent Magnetic 

 Behavior of the Core of a Large Electromagnet in the 

 Coil of which a Current is growing. 



If after the solid core of a large electromagnet had been demagnetized 

 we were to establish a steady current in the exciting coil by applying 

 to its circuit a constant electromotive force, eddy currents would, of 

 course, be set up in the core, and at any instant during the growth of 

 the current in the coil the iron at the centre of the core would be sub- 

 jected to a magnetic field weaker than the field belonging to a steady 

 current of intensity equal to the instantaneous strength of the coil 

 current. If, therefore, we were to attempt to determine the magnetic 

 properties of the core from the record of an oscillograph in the coil cir- 

 cuit, we should find that the induction through the core corresponding 

 to a given instantaneous current intensity in the coil was less than 

 the flux belonging to a steady current of the same intensity as deter- 



