( 667 ) 
so that the tube moves with slight torsion in them. A little cotton¬ 
wool placed on the bottom of the vacuumglass and attached to the 
holder lessens the sudden bubbling 1 ) up of the hydrogen 1 ). 
Further additions of liquid hydrogen are made in the same way 
as the first. As a rule various series of measurements could be made 
with a single filling with hydrogen. The point of the vacuuwglass 
which could not be silvered was protected by a small silvered 
vacuum beaker L containing liquid air. When the portion of the 
apparatus above the diaphragms B l0 is again at ordinary temperature 
after a filling with liquid hydrogen, one can hardly notice that there 
is liquid hydrogen in the apparatus at all, if it is not above A s . 
In the course of time a little mist is precipitated on the vacuum tube. 
By surrounding the tube at with blotting paper, the moisture 
is prevented from trickling down between the pole-pieces. Further¬ 
more a stream of air is directed against the tube between the pole- 
pieces. Hence the pole-pieces are in no way affected by the cryogenic 
operations. 
/. The springs are phosphorbronze. This substance is non-magnetic 
and acquires very little permanent set. Springs of the same constant 
can be made by winding a spiral either of a thin short wire 
or of a much longer thicker one. Of the two, the one which 
has the greater mass will experience the smaller specific changes, 
and consequently will be the more perfectly elastic in working. This 
circumstance has been duly taken into account. The springs are provided 
with straight extensions in the direction of their axis and are connected 
with the holder and the rod k (fig. 3) by screws. The turns of the 
spirals do not touch each other. The temperature of the spring is 
measured by a mercury thermometer that is clamped against the 
cap D and with it is insulated with wool. The constants of the two 
springs used are 261000 and 22300 dyne-centimetres per radian. 
The corrections for the influence of the stretching wire and for the 
temperature change of the spring will be discussed in § 4. 
The ellipsoids of iron, nickel and cobalt are 3 mm. long and 
1,333 mm. thick. They have been made with great accuracy by the 
Societe Grenevoise pour la Construction d’Instruments de Physique. 
They were turned under a microscope giving a 30-fold magnification 
and provided with a camera lucida so that the image of the object 
and an enlarged drawing could be superposed. Measurements with 
0 Should this occur one must ensure that the oil 'of the damper is not cooled 
by the drops that are thrown up. 
