.1890—91.] Prof. C. G. Knott on Magnetic Strains. 317 
For fields higher than 50 the following remarkable results were 
obtained 
Field. 
A + 2 p 
r 
60 
- 97 x 10 -7 
- 100 x 10~ 7 
+ 457 x 10~ 7 
100 
-8-0 „ 
-140 ,, 
+ 66 
135 
0 
-163 „ 
+ 81-5 „ 
240 
+ 4*0 „ 
-190 ,, 
+ 97 
260 
1 
+ 9*0 „ ? 
-202 „ 
+ lOo ‘5 , , 
Thus, for the iron tubes, the transverse contraction always exceeds 
the longitudinal extension, so that there is on the whole a diminution 
of the internal space. There is evidence of the contraction attaining 
a maximum, which, in the case of the thinner walled tubes (A 2 and 
A 3 ), occurs in a field not far removed from the field which produces 
the maximum extension. 
For the nickel tube, the transverse expansion differs so slightly 
from the longitudinal contraction, that the change of volume of the 
internal space, though of the same order of quantity as the correspond- 
ing change in the iron tubes, is a very small fraction indeed of the 
change that would result from the longitudinal contraction acting 
alone. In low fields, the longitudinal contraction overbalances the 
transverse expansion, causing a compression. This compression 
reaches a maximum about field 60, and then falls off first slowly, 
then more rapidly. About 140 it becomes zero, and changes 
sign in higher fields. In field 260 a very distinct dilatation is 
produced about equal to the maximum compression obtained in 
field 60. The rapid changes of temperature of the liquid in the 
heart of the magnetising coil, when the high currents were used, 
made accurate measurements of the changes of volume impossible ; 
but there was no doubt as to the fact of the change of sign in the 
compression when the field was taken high enough. 
It should be mentioned that, in an experiment with a glass tube 
substituted for the iron or nickel tube, no effect was produced ; so 
that the alcohol itself was uninfluenced by the magnetising force. 
An experiment was also tried with a current of seven amperes passed 
along the iron tubes, so as to cause a circular magnetisation of the 
outer circumference. No change of volume was observed, however, 
probably because of the comparative smallness of the fields involved. 
