216 Proceedings of Royal Society of Edinburgh. [sess. 
The Strains produced in Iron, Steel, Nickel, and Cobalt 
Tubes in the Magnetic Field. Part II. By Professor 
C. G. Knott, D.Sc., F.R.S.E. 
(Abstract.) 
(Read June 6, 1898.) 
The most important part of the paper has to do with the be- 
haviour of certain iron and nickel tubes, considerably shorter and 
narrower than those discussed in Part I.* Two iron bars, A and 
B (A being twice the length of B), and a nickel bar, B, were bored 
out by successive stages, so as to give three series of tubes of in- 
creasing bore. Each tube was subjected to four distinct experi- 
ments. These were — 
1. Measurements in various magnetic fields of the correspond- 
ing changes of volume of bore. 
2. Measurements in the same fields of changes of length. 
3. Measurements in the same fields of changes of volume of the 
material of the tube. 
4. Measurements in the same fields of apparent external changes 
of volume, the tube being plugged and treated as a bar. 
Erom these measurements the principal coefficients of strain at 
the inner and outer walls were determined. An element originally 
spherical became changed into an ellipsoid, whose principal axes 
were parallel to the axis of the tube, parallel to the radius of the 
tube passing through the element, and perpendicular to these two 
directions. The corresponding coefficients of strain are distin- 
guished as the longitudinal elongation A, the radial elongation v, 
and the tangential elongation, p. 
In nickel A is always negative, and p and v generally positive.. 
In the narrowest bored tubes p is occasionally negative. In the 
B-tubes A reaches a value of nearly - 25 x 10 -6 in field 500 (C.G.S. 
units) ; p almost touches the value + 7 x 10 -6 ; and because of the 
* See Trans. It.S.E. , vol. xxxviii. pp. 527-555 (1896). 
