PEIRCE. 



ESTABLISHMENT OF CURRENT IN COIL. 



157 



It is difficult to obtain an accurate hysteresis diagram for trans- 

 formers containing massive iron cores, by the "step-by-step" ballistic 

 method, with a short period galvanometer. Although, eddy currents 

 may be nonexistent the time-lag of magnetization necessitates the 

 use of a long period instrument, when ballistic methods are employed. 

 The galvanometer used throughout these experiments was of the 

 d' Arson val type, and had a period of about four minutes. The 

 accuracy of the ballistic method was tested by comparing the results, 

 thus obtained, of a corresponding hysteresis cycle for an excitation of 



MAXWELLS. 



10,000 AMPERE TURN 



Figure 4. Hysteresis diagrams for the core of the electromagnet J. 



1812 ampere turns, with a series of results reduced from oscillograph 

 records. Throughout the comparison the agreement lay within a 

 tenth of one per cent, and this may be considered close since it is not 

 always possible to make a large mass of iron travel exactly the same 

 magnetic journey twice. 



Computation of Current Growth on Reversal of E. M. F. — 

 Figure 4 shows a series of hysteresis diagrams for the electromagnet 

 J obtained by decreasing and reversing the exciting current step by 

 step after maximum excitations of 1812, 5370, and 10880 ampere-turns. 

 The results of measurements of the flux changes in the core for the 



