710 
SIE WILLIAM THOMSON ON THE 
Bs, and (§ 191) subtracting 20 from each number, we see that the effect on the mag- 
netization of the wire producible by repeating M(— ) and B over and over again without 
other disturbance would be about 16 each way with 84 lbs. on, and probably between 
5-g- and 9 with 28 lbs. on. (This agrees in kind with the conclusion deducible from the 
comparison between the previous Series II. and VI. ; but the absolute magnitudes of the 
results seem smaller, probably because of less battery-power in those series than in 
Series XXXI.) The whole effects of the M( + ) after M( — ) and B, and of the M( — ) after 
M(+) and B, were still, as in Series XXX., each equal to 180 or 181, with only the 
28 lbs. on, and therefore (subtracting 20 for the induction from coil to coil) we had 160 
or 161, say 160^-, for the sum of the demagnetization and reverse magnetization produced 
by a M following a B and a previous M of the opposite direction. Now came, in the 
course of a few minutes, a most startling discovery. With the 84 lbs. on, the sum of 
magnetization and demagnetization produced by the M after B from previous opposite 
M was 130, or less by 30|- with those than with the 28 lbs. Thus we see that while 
the magnetic effect of stopping the current is greater , the effect of subsequently insti- 
tuting the current in the reverse direction is less , with the heavy than with the light 
weight ; and less by about three times as great a difference. 
193. Consider, lastly, the effect of a sudden reversal of the current. This may be 
regarded as the sum of the effects of stopping it, and starting it in the reverse direction ; 
and therefore may be expected to be less with the 84 lbs. than with the 28 lbs. by about 
-§ of the difference between the effects of starting the reverse current with the heavy 
and with the light weights hung on the wire. This inference is verified in the con- 
cluding thirteen results of the series before removal of the steel wire. Thus (§ 190) 
subtracting 40 for the induction of coil on coil in the reversal, we find 140, 139-^, 141, 
136, 140, 139, 139 for magnetic effects of reversals with 84 lbs. hung on the wire, of 
which the mean is 139*2 ; and 159, 158, 159, 160, 160, 160, for the magnetic effects 
of reversals with 28 lbs. on, of which the mean is 15 9 ’3. 
194. After the conclusion of these experiments on the steel wire, I made many 
experiments of the same kind on soft-iron wires of various qualities substituted for it 
in the same apparatus, and I have obtained results of the same kind, as to the effects of 
hanging on and taking off weights, while the magnetizing current is kept flowing. I 
have also obtained some very remarkable and perplexing results by putting weights on 
and off with the current not flowing. In one of the iron wires the effect found was 
opposite to that in steel ; that is to say, putting on weight augmented and taking off 
weight diminished the residual magnetism ; in another the same effect as in steel was 
found, that is, putting on diminished and taking off augmented the residual magnetism. 
‘Neither of these was as soft as some of the other wires tried, and the one (“ bright soft 
iron wire,” Johnstone’s) that agreed with steel was remarked on at the time as much 
harder than another that had been previously experimented on (“ black soft iron wire,” 
J ohnstone’s). This latter seemed utterly destitute of retentive power under the influence 
of putting the weights on and off. Like all the others it always experienced a diminution 
