586 Proceedings of Royal Society of Edinburgh. [sess, 
Magnetic Twist in Nickel Tubes. By Professor C. G, 
Knott, D.Sc. 
(Read February 20, 1899, ) 
(1.) Introduction . — In previous papers* I have had occasion to 
discuss at some length the relations between Joule’s discovery of 
the elongations accompanying magnetization in iron and Wiede- 
mann’s discovery of the twist produced in an iron wire under the 
combined influence of longitudinal and circular magnetizing forces. 
Following out a suggestion of Maxwell’s f as to the intimate 
connection between these two phenomena, I turned my attention 
to nickel, in the hope of finding the Wiedemann effect in it 
opposite to what it is in iron in low magnetic fields. The fact 
that, as Barrett J had shown, nickel contracted in length when 
longitudinally magnetized, whereas, in low fields, iron elongated 
was the ground for this hope, which experiment fully justified. 
In a recent paper on the strains produced in iron, steel, nickel, 
and cobalt tubes in the magnetic field {Trans. Roy. Soc. Rdin., 
vol. xxxix., 1898), I have obtained, on sufficiently reasonable 
assumptions, values for the strain coefficients at the inner and 
outer walls of these tubes. Hitherto, work by other experimenters 
on similar lines had been on ellipsoids, rods, or wires, solid through- 
out. The theoretical discussion of the Wiedemann Effect (as I 
have called it) was rendered the more difficult on this account, 
and also because the current producing the circular magnetization 
flowed through the magnetized material. In my paper of 1888 
referred to above, I make an attempt to get a formula by means 
of which the Wiedemann effect may he calculated from the Joule 
effect. The formula is applicable, however, only to a thin-walled 
* Trans. Boy. Soc. Edin., vol. xxix. (1883) pp. 193-203 ; vol. xxxv. (1888) 
pp. 377-390 ; vol. xxxvi. (1891) pp. 485-535 ; Phil. Mag., January 1894. 
+ See Electricity and Magnetism (2nd edition, vol. ii. p. 448) ; see also 
Chrystal’s article “ Magnetism ” in the Encyclopaedia Britannica (vol, xv„ 
pp. 269-271). 
$ See Nature , voh xxvi. , 1882,. 
