( 805 ) 
is introduced into the protecting tube through the small tube P fi 
and into the inner tube through Q s . The two tubes P, and Q t lead 
the vaporized oxygen through the valves P 8 and Q a (tig. 2) to two 
gasometers. Two manometers P e and Q s , the latter of which is 
provided with an indicator Q 7 so that small vapour pressures may 
be read off accurately, serve at the same time as safety valves. It 
is not necessary that the oxygen in the protecting tube should be as 
pure as that in the inner tube; for the latter, with which the obser¬ 
vations were made, very pure oxygen was used. 
A double sliding movement R allowed an easy adjustment of the 
apparatus each time, so that the meniscus in the measuring tube 
just reached the desired point in the field between the poles, usually 
in the axis of the pole pieces. 
Course of a series of measurements. The field is brought to the 
desired strength and by means of R the meniscus is made to rise to the 
desired point, which is read off on a small scale. Then the meniscus is 
moved up and down several times while care is taken that the field slowly 
increases. In this way the temperature is made everywhere the same 
and the walls of the tpbe are wetted. While the field has the desired 
value the position of the meniscus is read off; then a reading is 
made while the field is off; after the meniscus has been three times 
allowed to rise somewhat higher than the desired position, another 
reading is made while the field is on; once more a reading is made 
with the field off and so on several times. In this manner the error 
arising from vaporization of the liquid during the adjustment of the 
cathetoineter is eliminated 1 )- It is not essential to know the position 
of the level in the other limb of the tube; so as to be able to take 
account of this, we ascertained the ratio of the cross-sections of the 
two limbs of the tube. 
We have further made sure that the residual magnetism exerted 
no appreciable influence upon the position of the meniscus after the 
current was cut off. For this purpose a feeble current was sent 
through the coils in the direction opposite to that which had just 
been broken. Had the residual field exerted any appreciable influence 
we should have seen first a further sinking of the level and 
then a rise as the current was slowly increased. This has not been 
observed. * 
We used the same electro-magnet as was used for the cryogenic 
investigation of the ferro-magnetic metals*), to which we must refer 
i) To control the position of the —-- 
measured the quantity of gas vaporized (cf. preceding page). 
*) P. Weiss and H. Kamerungh Onhes, 1. c. 
