without reading the thermometer, the thermometer is still necessary, 
however, to indicate the position of the upper surface of the liquid 
gas which is no longer visible beneath A s . As soon as the level 
sinks below the upper end of the reservoir 6 , of the thermometer, 
the mercury in the stem sinks. 
d. First the electromagnet is adjusted which operation is independent 
of the centring of the adjusting tube, the holder and the vacuum 
tube. The axis round which it turns is made vertical, and then the 
pole distance is centred round this axis. Next the centre of the truncated 
spherical socket 6r 10 (fig. 1, 2 and 3) is made to coincide with the axis 
round which the magnet turns, It is supported by a plate which is 
attached by two beams to the freestone pillar ©. The cryogenic 
apparatus is then brought from its auxiliary support and arranged 
in its proper position by placing the ball-shaped portion of the surface 
of the ring B x in the concentric socket G 10 ; the centring of the 
narrow portion of the vacuum tube on the turning-axis of the magnet 
is completed by means of wing nuts on the ring B t . This centring 
must be done with great accuracy, for the magnet must turn freely 
and the distance between the vacuum tube and either pole is not 
more than half a millimetre. It can, however, easily be accomplished 
e. Liquid hydrogen is introduced into the apparatus by a german 
silver tube B , r (cf. Comm. N*. 94/). The gas formed by evaporation 
escapes through B 8 (figs. 3 and 1) and through the valves ^, K, (fig. 1) 
to a gasometer or to a vacuum pump. By means of the valves the 
vapour pressure is regulated, and its value is read on a manometer 
^ which at the same time acts as a safety valve. In experiments 
made m the neighbourhood of the melting point of hydrogen the 
pressure was kept slightly above that of the triple point, 
i intr0d , u i cin ^ hquid hydrogen through the tube B 7 , which is 
c osed by a rubber tube with a glass stopper, the air is pumped out 
of the apparatus through the valve K It is absolutely essendal that 
he apparatus should be air tight, for traces of air would solidify 
Zn^ 'IT J and ’ owin « to ma g ne tic attraction, would 
collect in the neighbourhood of the ellipsoid. 
containing & th f* 6 * C °°^ n ? °* u PP er portion of the apparatus 
containing the torsion spring by the boiling hydrogen a number of 
large openings are made in the tube / " ‘ 
Wav that nn . lUDe J* 3 ) arranged in such a 
this Conner screens * ^ r ° ltS resistance to torsion. In addition to 
this copper screens surrounding /. and soldered to B„ are arranged 
