093 



/'^ = ciiu If, vvliere /,„ and if i'e|)reseiil llic ciirroiils in llic inoNuhlc 

 and fixed coils and c i.s a constant whicli is determined once for all 

 b}' using known forces (weights) ^). 



The electromagnetic compensalioji has for its sole object making 

 a balance with the forces to be measured : the weight of the carrier 

 itself is balanced Jiydrostatically bj' means of two floats immersed 

 in mercury; the principle is therefore similar to that of a constant- 

 volume hydrometer. 



§ 3. The various parts of tlie apparatus. 



a. Ihe enclosure of the cri/ostat. The space inside tl»e enclosure 

 A of the apparatus is divided by screens E, which prevent exchange 

 of lieat between the two parts. The cryogenic part below the screens 

 contains everything connected with the establishment of low tem- 

 peratures, in the chamber above the screens which remains practi- 

 cally at constant temperature, all the delicate parts for the measure- 

 ment of the forces are bi-ought together. 



The wall of the cryogenic space below E is of german silver. It 

 is joined airtight by means of the india-rubber ring A^ to the 

 vacuum glass A^ which contains the bath of liquid gas. The liquid 

 gas is supplied by the german silver tube Ar,, the vapours are 

 carried off bj^ A^. The steel capillary of a heliumthermometer Tli. 

 is soldered through the wall of the cap. 



The upper part of the vacuum-glass is comparatively wide (6 cms), 

 so that the liquid level falls very slowly during the evaporation, 

 which as we shall see is of importance. By means of the copjier 

 ring A^ and the rods A^ the vacuumglass is firmly connected to 

 the cap, that the considerable forces arising from changes of internal 

 pressure may not change its position. 



1) For keeping in equilibrium an apparatus of the general type under consider- 

 ation a;ny kind of force may be used which can be changed gradually without 

 touching the carrier. We can thus work equally well with a given compensating 

 force (definite weights) and changing field (regulating the current through the 

 electromagnet) as with a given field and changing electromagnetic compensating 

 force. When our apparatus was first constructed we did not possess the necessary 

 appliances for accurate field-measurements, and in examining the dependence of 

 the phenomena on the temperature we had to make ourselves independent of the 

 change of the field by confining ourselves to the investigation of the magnetisation at 

 different temperatures at a few field-stenglhs chosen beforehand and kept constant 

 each time during the experiments 



For a modification in which the electromagnetic compensation is replaced by 

 a compensation by weights we refer to a forthcoming description of the appa- 

 ratus which was used in their investigations by Kamerlingh Onnes and Oosterhuis 

 (Comm. nO. 1296 etc.). 



