is necessary that the total quantity of liquid may not appreciably 
alter during the measurement of one rise. Moreover magnetic 
action itself increases the difficulties; it is easy to see that it can 
occasion the formation of gas-bubbles which divide the column of 
liquid into two parts, so that the one portion remains suspended 
between the poles, while the other falls back again. In that case 
measurement of the ascent is out of the question. 
Starting from the thermodynamic potential it appears that in every 
case the relation 
2 gy Q >K{H'-H y ') 
must hold, where H is the field at the surface of the liquid, and 
H y the field at a distance y below the surface of the liquid. These 
conditions shew that there is a limit to the intensity of the fields in 
which measurements may be made, for they necessitate a range of 
extended fields (in this case in a vertical direction). Conical pole- 
pieces are thus as a matter of fact barred. 
After several preliminary experiments an apparatus was constructed, 
the most important part of which consisted of two concentric double- 
walled vacuum tubes, with which we already succeeded in obtaining 
rather successful measurements. The walls of the double vacuum 
tube were not silvered, so that we were able to watch how the 
liquid behaved during the experiments. From the experience thus 
acquired the improved apparatus which we shall now proceed to 
describe was designed and constructed. 
It will be seen that the construction of the apparatus lays a very 
heavy tax upon the art of the glass-blower*). As before, the chief 
part consisted of two independent U-shaped vacuum tubes, the one 
fitting inside the other. The double walls of each tube are completely 
silvered on the vacuum side, except in the case of the inner tube, 
where the distance which the liquid ascends is left free, and in the 
outer where a sufficient length is left unsilvered to leave a strip of 
a few millimeters breadth through which the level of the liquid can 
be read. One of the tubes completely surrounds that portion of the 
other which contains liquid; this we call the protecting tube. The 
narrowest portion M t (fig. 2) is placed between the poles of the 
electro-magnet. The narrow limb of the inner tube must of course 
be perfectly cylindrical. The other limb is enlarged and serves as a 
reservoir. In order to be able to apply equation (2) all care was 
taken that the temperature of the liquid and vapour up to a height 
vacuum tube was prepared by Mr. Kessklring, Laboratory glass- 
remainder by Mr. Flw, technical assistant at the Laboratory. 
