1890-91.] Prof. C. G. Knott on Magnetic Strains. 
315 
On the Effect of Longitudinal Magnetisation on the 
Interior Volume of Iron and Nickel Tubes. By 
Professor Cargill G\ Knott, D.Sc., F.R.S.E. 
(Read July 20, 1891.) 
The following results in magnetic strains are, so far as I am 
aware, new. They supplement in an interesting way Joule’s old 
result of no change of volume in an iron rod when it is magnetised. 
What is given here is only preliminary, and suggests many lines of 
research which I hope to be able to follow out later. 
The broad fact established is, that the internal capacity of certain 
iron and nickel tubes alters appreciably when the tubes are 
magnetised longitudinally. The tubes were 34’8 cm. long, and 
were all about 3 cm. external diameter. One iron tube had an 
internal diameter of 1 cm., and another of 2 cm. These I shall call 
and A 2 respectively. A 3 represents the third iron tube, whose 
wall was about 1 mm. thick. The nickel tube (B) had its wall 03 
mm. thick. When experimented with, each tube was tightly corked 
at both ends, and through the one cork a fine capillary glass tube 
projected. The tube was filled with alcohol coloured with cochineal. 
The changes of volume were measured by the movement of the 
end of the liquid column in the capillary tube. This was viewed 
through a microscope. A movement in the tube through a distance 
equal to one division of the microscope micrometer meant a change 
of volume of 7'2 x 10 -6 cub. cm. 
As an example, take the case of A 1? the small bored iron tube, in 
a field of 250. The sudden outward movement of the liquid 
meniscus showed a total compression (change per unit volume) of 
21 x 10 -7 in the region inside the tube. But we know that in this 
field ordinary wrought iron lengthens ; and in virtue of this lengthen- 
ing the internal volume will be increased. It is clear, then, that 
the transverse contraction of the walls of the tube has overbalanced 
the longitudinal extension. If A. /x represent the elongations parallel 
to and perpendicular to the axis of the tube along the inner surface 
of the bore, the dilatation will be A, -i- 2/x. Now, in field 250 Bidwell 
