316 Proceedings of Royal Society of Edinburgh. [sess. 
finds A = + 5 x 10' 7 ; hence at once p = - 13 x 10 7 . I give a few 
results for the different tubes in various fields. 
In Field 50. 
Tube. 
X + 2/x 
observed. 
A 
Bid well. 
V- 
calculated. 
A x 
- 1*84 x 10 -7 
+ 10 x 10 -7 
- 5*9 x 10- 7 
a 2 
-2T 
? ? 
- 6T x 10 -7 
a 3 
-1*2 
-5-6x10-7 
In Field 125. 
Tube. 
A + 2 fJL 
A 
r- 
A, 
- 6 "6 x lO -7 
+ 18 x 10 -7 
-12-3x10-7 
A 2 
-8-4 „ 
? 5 
-13-2x10-7 
As 
-3 
-10-5x10-7 
In Field 250. 
Tube. 
A + 2 ix 
A 
-21 x 10~ 7 
+ 5x10-7 
i 
00 
X 
o 
<1 
A., 
- 7 x 10 ~ 7 
5 9 
- 6 x 10 —7 
a; 
- 2-6 ,, ? 
- 7*6 x 10~ 7 
Unfortunately I possessed no nickel tubes shaped like the iron 
ones, so had to content myself in the meantime with a thin walled 
tube formed by rolling up a sheet of ordinary commercial nickel to 
the convenient size. The results for this tube were of great interest. 
Up to a field of 50, the compression of the inside space varied 
uniformly with the field, the dilatation being given by the formula 
A+2/* = - l - 8 x 10 _8 H, 
where H is the longitudinal field. Now BidwelFs results give up to 
the same field the following expression for A: — 
A = -18 x 10 _8 H. 
Ijl= + 8-1 x 10“ 8 H. 
Hence 
