PEIRCE, — BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 125 



direction the core had arrived at the point Q. This process, repeated 

 for many points on the line GPV, yielded the curve VQACG. If 

 after being at V the core was brought to a point between P and iV, 

 and if after it had been many times reversed the current was decreased 

 by short steps with many reversals at each stage, the core traversed 

 the curve U, whereas if the first drop carried the core no farther than 

 P, the procedure led the core to the origin along the curve /. The 

 lowest point of the curve VQA G lies, of course, nearly over the point 

 Z. The shaded diagram in the upper part of the figure shows a 

 similar curve obtained at another time and drawn strictly to scale. 

 If after many reversals of a comparatively small current the core which 

 started at L reached the point F, and if the current was then slowly 

 increased, the core made the journey indicated by the line FL. The 

 shaded diagram in the lower part of the figure is a reduction of a curve 

 obtained with a large induction coil the core of which is a compact 

 round bundle of fine wire 7.5 cms. in diameter and about 85 cms. long. 

 The curves oec, cak, cek, in this diagram correspond to OIQ V, VPG, 

 VQAG in the larger figure. The retentiveness of a core of these 

 dimensions is, of course, very small. 



Even if much time has been spent in demagnetizing a large closed 

 core by sending through the exciting coil currents alternately in one 

 direction and in the other, of intensities gradually decreasing to a very 

 small final value, it frequently happens that after a much larger 

 current has been put for, say, twenty times through the coil alternately 

 in one direction and the other, the hysteresis cycle does not "close," 

 for the change of flux caused by applying the given current in one 

 direction is not equal to the flux change caused by applying the same 

 current in the other. This fact often makes the accurate determina- 

 tion of a hysteresis diagram for such a core a long and trying piece of 

 work. Some toroidal cores I have never succeeded in demagnetizing 

 completely. The demagnetizing apparatus which I have usually 

 employed in the course of the work here described consists first of a 

 storage battery of forty large cells, a set of rheostats made up of 

 metallic and liquid resistances intended for heavy currents, and a 

 commutator run from the main shaft of the laboratory machine shop, 

 and so arranged as to reverse the direction of the current from the cells 

 every ten seconds. Starting with no resistance in the rheostats, 

 resistance was gradually introduced into the circuit until the current 

 had become very small. After this procedure, the secondary circuit 

 of a specially constructed transformer was attached to the exciting coil 

 of the magnet, and from an initial voltage of about 660, at 60 cycles 

 per second, the electromotive force was gradually decreased until the 



