RESEARCHES IN" MAGNETISM. 
535 
§ 14. Another cause contributes to produce the erroneous opinion that soft iron has 
little retentiveness, namely, the enormous influence which the slightest mechanical 
disturbance has in removing the residual magnetism. The soft iron ring of fig. 2 loses 
nearly all its residual magnetism if gently tapped after the magnetising force has 
ceased to act. I have frequently removed in this way all but one or at most two per 
cent, of the originally large residue. So susceptible indeed is soft iron to the effect of 
mechanical disturbance, that when the field is removed the lightest touch by the 
fingers suffices to destroy a large part of the residual magnetism, although it appears 
that so long as the iron is left perfectly undisturbed the residual magnetism does not 
suffer loss with lapse of time. I shall recur to the effects of vibration later ; mean¬ 
while they are mentioned in order to explain that we may take advantage of this 
property to reduce a piece of soft iron to a very nearly neutral state after magnetisa¬ 
tion, so that we may study the comparative effects of again magnetising the same 
piece under different conditions. Simple tapping in a field of no force reduces the 
piece to a state differing very little from that in which it was after annealing and 
before any magnetisation had taken place. This I have found to be the case by 
taking curves of magnetisation of a very soft iron wire after annealing, and again 
after shaking out the residual magnetism : such curves turn out to be sensibly 
coincident. 
§ 15. Magnetisation of long Rods of various lengths .—-Taking advantage of this 
property, in virtue of which we can reduce a given specimen of soft iron after 
magnetisation to a state nearly identical with its primitive state, I made the follow¬ 
ing group of observations to illustrate the influence of length on the magnetisation 
of straight rods of circular section. A long straight wire of the same very soft iron 
as the ring of fig. 2 was well annealed, wound with a magnetising solenoid and placed 
on a horizontal table in the E.-W. position. After the wire was tapped to get rid of 
its initial magnetism a curve of 23 and -§ was taken by the ballistic method. >§ was 
then reduced to zero and the residual magnetism was shaken out by tapping the wire 
on the table, still keeping the E.-W. position, and the permanent residue (which was 
very small) was determined by slipping off the induction coil. The induction coil was 
about 5 centims. long, and was placed at the centre of the rod’s length. The process 
of magnetisation was repeated three times, giving sensibly the same results each time. 
The wires diameter was 0T58 centime, and its original length was 47'5 centims., or 300 
diameters. Then the length was reduced to 31'6 centims., or 200 diameters, by cut¬ 
ting off equal portions from both ends, and a curve of magnetisation was taken. The 
residual magnetism was again tapped out, the length again reduced in the same way 
to 150 diameters, and another curve of magnetisation taken. The same process was 
repeated with the length equal'to 100, 75, and 50 diameters successively. The 
peculiarity of the method lay in this, that throughout the series of observations the 
magnetic induction was determined through precisely the same piece of material. 
The influence of length per se w 7 as therefore exhibited in a way which would have 
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