fi95 



c. Vertical (juldance of the carrier. The space between Hie 

 experimental objects and the inner wail of the vacnum-glass can 

 sometimes not be more than a few tenths of a millimetre when 

 measurements with strong fields are to be made; with the slightest 

 movement of the axis of the carrier from its original position owing 

 to a small asymmetry in the action of the electromagnet or any 

 other cause the carrier would not be able to move up and down 

 freely. This difticulty was quite satisfactorily overcome by guiding 

 the carrier in its up and dowqi motion by the aid of tw^o flattened 

 spiral springs '). The outer end of both is fastened to the stationary 

 part of the apparatus, the inner end to the carrier and the plane 

 of the springs is placed at right angles to the axis of the carrier. 

 By the device of using flat springs a movement of the middle in 

 the plane of each of the spirals is ahiiost completely prevented. 

 Usually the upper spring R^ attached to the carrier remains the 

 same, while each separate experimental object is provided with its 

 own spring, which is remo^'ed from the apparatus with the object. 



cl. The liydrometric equilibrium. To keep the carrier afloat on 

 mercury the upper chamber of the apparatus is provided with a 

 ring-shaped trough Q (in our experiments of glass, later on of china) 

 which is centred on the axis of the carrier. The latter is fitted with 

 a horizontal arm F^, in which at both ends are fixed the tubes of 

 the floats F^, glass bulbs, the shape of which is not unlike a flattened 

 ellipsoid. The tubes of these bulbs which are of very small section 

 are the only part that projects above the mercury. The section has 

 to be small in order rhat the upward pressure of the mercury shall 

 vary very little, if the apparatus is to be sensitive to a very small 

 change of the vertical force acting on it. But the size cannot fall 

 below a certain limit, because the tubes must also serve to com- 

 pensate the diminution in upward pressure in the bath on the ex- 

 perimental object, owing to evaporation. 



This compensation is effected by raising the level of the mercury. 

 For this purpose use is made of a plunger D.^, a small glass flask 

 of a shape corresponding to that of the trough which is moved up 

 and down by means of a rod D^ with thread and milled head D^ 

 passing through a stuffing box Dg. This contrivance, which was found 



1) Springs of that kind are made by cutting on the lathe a spiral groove 0.2 

 to 0..3 m.m wide in a plate of german silver (comp. perspective figures R^E^). 

 By giving different widths to the spiral strip for a given diameter springs may be 

 obtained of any desired degree of sensibility. The inner end is soldered to a small 

 tube, the outer end is fixed in a clamping screw. 



