ON THE MEASUREMENT OF MAGNETIC HYSTERESIS. 
71 
The experiments of Professor J. A. Ewing* on soft iron show that after the few 
dozens of reversals which are necessary to avoid the effects of the initial diminntion 
of Bq, the values of and of W remain constant, even after 70,000,000 of cycles 
of reversal when the temperature of the iron is not allowed to rise above that 
of the atmosphere. In his experiments the maximum magnetic force in these 
cycles was about 4, and the maximum induction, Bg, about 8000. It is now well 
known that if iron be raised to a temperature only 50° C. above the atmospheric 
temperature, and be maintained in that state for many hours, a large increase in the 
hysteresis often results. 
§ 47. Soft Iron demagnetised hy Annealing .—The specimen was a wire of soft 
iron '0265 sq. centim. in section. In each case it was annealed before the observa¬ 
tions for any given value of Hq were made ; the previous magnetic history of the iron 
was thus wiped out. The key of § 31 was used. 
No. 
Ho = 
2-50. 
' Ho = 
4-98. 
Ho = 
7-57. 
Ho = 
10-86. 
Ho = 14-70. 
Bo. 
W. 
Bo. 
W. 
Bo. 
Bo. 
W. 
Bo. 
AY. 
1 
2220 
598 
8150 
5250 
10550 
9500 
11800 
124.50 
12840 
14610 
2 
2040 
519 
7970 
4870 
10430 
8980 
11680 
12050 
12830 
14340 
3 
2000 
488 
7950 
4730 
10340 
88.30 
11660 
11960 
12760 
14230 
11 
1940 
452 
7740 
4530 
10250 
8520 
11660 
11850 
12730 
14140 
21 
1890 
440 
7700 
4420 
10250 
8590 
11670 
11760 
12720 
13990 
41 
1840 
433 
7650 
4360 
10220 
8450 
11660 
11760 
12730 
14070 
No. 
Ho = 
20-05. 
Ho = 
25-15. 
H„ = 
30-24. 
Ho = 
40-65. 
Bo. 
AY. 
Bo. 
AY. 
Bo. 
AY. 
B, 
AY. 
1 
13480 
17420 
13270 
20020 
14360 
21300 
14760 
22600 
2 
13430 
17300 
13260 
198.50 
14300 
21260 
14740 
22600 
3 
13420 
17200 
13280 
19770 
14300 
20940 
14720 
22610 
11 
13420 
17060 
13230 
19690 
14290 
20820 
14720 
22400 
21 
134.30 
17100 
13280 
19720 
14290 
20900 
14730 
22560 
41 
13420 
17080 
1.3250 
19640 
14310 
20820 
14710 
22630 
It will be seen that the effect of continued reversals of Hq in producing a diminu¬ 
tion of Bq and W rapidly decreases as Hq increases, both Bq and W becoming sensibly 
constant when Hq reaches the value 40'6 5. The following table illustrates this fact, 
and shows that the percentage change of W is always greater than the percentage 
change of Bq. The change recorded is that which occurred in the first 41 cycles 
* ‘ The Electrician,’ vol. 34, January 11, 1895. 
