466 Mr. W. E. Sumpner on the Variation 



point on the ascending curve of magnetization, and P' any 

 point on the descending curve, the slopes PA and P'A' cor- 

 respond with changes of current in the same direction as 

 those immediately preceding, and these slopes are steeper 

 than those of PB and P'B', which correspond with reverse 

 changes of the magnetizing force. 



If the current be changed between the two values cor- 

 responding with the points P and A, and the swings of the 

 ballistic galvanometer observed for each alternation, the first 

 fling will correspond with the slope PA, if the magnetization 

 has been increased up to the point P. All the succeeding 

 swings will, however, correspond with more gradual slopes, 

 AC, because the magnetization owing to hysteresis will not 

 return to the state P, but will proceed around a Ewing's 

 cycle between the two points A and C, if the current be 

 alternated between the two values considered. In the case of a 

 transformer, two or three successively diminishing swings were 

 often observed before the changes became cyclic. Whether on 

 the ascending or descending portions of the magnetization - 

 curve, the first fling should be greater than the succeeding 

 ones if it corresponds with a change of current of the same 

 kind as that immediately preceding. 



The following experiments on the Ferranti field-magnets 

 illustrate these remarks : — 



Previous history. 



Current Change. 



Successive Swings. 



increased to +12 amperes, 

 and diminished to 5-86. 



5-86 to 3-56 

 3-56 to 586 



-232 -205 

 +193 +192 



C diminished to 0, 

 and raised to 356. 



3-5fi to 5-86 

 5-86 to 3-56 



+230 +197 +195 

 -209 -205 



C increased to 4'86, 

 and diminished to 3'13. 



3-13 to 486 

 4-86 to 313 



+ 149 +149 

 -151 -152 



increased to +13, 

 and diminished to 4 "86. 



4-86 to 3-13 

 3-13 to 486 



-180 -157 -157 

 + 153 +152 



C diminished to 0, 

 and increased to 3' 13. 



313 to 4-86 

 4-86 to 313 



+ 189 +153 +153 

 -162 -159 



The swings succeeding the first are seen to be all very 

 nearly equal. The negative swings are slightly greater than 

 the positive ones, because the current flowing is smaller, the 

 resistances in circuit larger, and the time-constant of discharge 

 smaller. Although the amount discharged is the same for 



