1898 - 99 .] Prof. Knott on Magnetic Twist in Nickel Tubes. 589 
angular displacement of the mirror was never greater than 1J 
minutes of arc. It was impossible to read certainly to less than 
tenths of a division. Consequently each number, although the 
average of eight readings, may be out by 2 or 3 in the second 
significant figure. 
These results are shown graphically in the full-line curves of 
fig. 1, p. 592. In these cases the axial current is kept constant, 
and the helical current is varied. When the helical current is 
kept constant, and the axial current varied, the twist is much 
smaller, being indeed barely measurable. Thus with a longi- 
tudinal field of 61, the establishing of a circular field of 5*74 gave 
a twist of only 1 x 10 ~ 6 . This agrees with my former results with 
wires. 
(3.) Circular and Longitudinal Magnetizations in the Nickel 
Tube . — In order to have as complete a knowledge as possible of 
the various magnetic properties of the tube, the magnetic induc- 
tions were carefully determined by the ballistic method, and the 
magnetizations deduced in the usual way. To the measurements 
made with the ballistic galvanometer corrections had to be 
applied for the lines of induction in the air spaces not occupied 
by the metal. In the case of the circular magnetization this 
correction was comparatively small ; but in the case of the 
longitudinal magnetization it amounted to a considerable fraction 
of the whole measured induction — to fully one-half in field 246. 
To make sure that the ends of the magnetized nickel tube had an 
inappreciable effect on the field inside the tube, this field was 
measured directly by means of a secondary coil inserted within 
the tube. It was from these measured inductions in part of the 
air space within the tube that the correction for the whole air 
space was calculated; but it was found that this correction, based 
on direct measurement, agreed to form significant figures with the 
correction calculated on the assumption that the thin-walled nickel 
tube did not appreciably alter the field in the included air space. 
The magnetizations I and I' corresponding to the longitudinal 
field H and the circular field H' are given in Table III. R and R' 
are the corresponding residual magnetizations, and the columns 
headed R/I, R'/T give the ratios of the residual to the total 
magnetization. 
