334 
MR. C. CHREE ON THE EFFECTS OF PRESSURE 
that, while the first application of the tension still increased the bar’s magnetisation, 
the cyclic effect had changed sign. On his p. 91, in experiment 3, is given an 
instance where a soft iron bar, exposed to one of his strongest fields, lost magnetisa¬ 
tion on the very first application of the tension. From the data supplied, however, 
it will he found that the shock-effect of this first tension was still an increase of 
magnetisation. Similar cases are also referred to on his p. 97, but no data are given 
from which the sign of the shock-effect can be deduced. 
From A^illari’s experiments it obviously follows that a critical field, or intensity of 
magnetisation, must exist where the cyclic effects of tension vanish. The field in 
which the effect vanishes is called by Sir W. Thomson the “ Villari critical point.” 
He and Professor Ewing have made it clear that the field in which this phenomenon 
occurs depends on the amount of the tension which is applied and removed, and also 
on the existence of any permanent load. Professor Ewing also prefers to define the 
ViLLARi point by reference to the magnetisation existing in the rod, and not to the 
field producing it. Some of Villari’s steel bars behaved like iron, but in others 
a critical field, if existent, was lower than the lowest field he employed. 
ViLLARi also observed the fall in the residual magnetisation caused by the first 
tension after the break of the current. He further observed that after several 
tension cycles there appeared a cyclic change in the residual magnetisation. Without 
making any restriction as to the strength of the pre-existing fields, he lays down the 
law, that in soft iron the residual magnetisation in the cyclic state is greatest when the 
tension is “ on,” while in hard iron and steel, it is greatest when the tension is “ off” 
I can find no clue as to the actual strength of the pre-existing field or fields he 
employed. 
Fitting an iron core inside a coaxial iron tube of the same length, Villari found 
that in fields between certain limits, the core and the tube showed, in response 
to cyclic changes of tension, cyclic changes of magnetisation of opposite signs. The 
phenomena, as A^illari himself pointed out, are exactly in accordance with the view, 
stated in § 5, that the intensity of magnetisation is greater on the surface of a bar than 
in its interior. 
§ 9. Sir W. Thomson experimenting on a soft iron wire permanently stretched, 
and tlien exposed to cycles “on” and “off” of some weight causing no further 
permanent extension, found a critical field which was in general higher the smaller the 
weight employed in the tension cycles. Tlie critical fields obtained by the magneto¬ 
metric method—calculated presumably for the centre of the magnetising coil— 
averaged about 20 C.G.S. units, and the fields at which the cyclic effect was a 
maximum, were roughly about a quarter of the critical. 
By the ballistic method with an induction coil at the centre of a much longer 
magnetising coil, critical fields such as 6'5 C.G.S. units were obtained. This Sir W. 
Thomson apparently assigns to the experimental wire having its magnetisation 
greatest at the centre of the magnetising spiral, so that this portion of the wire would 
