1112 



ciilar to the brass capiHaiT a needle was soldered. When the apparatus 

 was mounted, so that it could be rotated, the needle passed a fixed 

 scale, indicating in this way the orientation oC the crystal axis in 

 the field. The resistances were measured with a Thomson bridge. As 

 this method requires wire resistances small compared with the fixed 

 resistance, in the arms of the bridge (110 ohms in each arm), the 

 above mentioned wires, which were of necessity thin because the 

 four of them had to pass through the glass and brass capillary, were 

 kept as short as possible. When they had left the brass capillary 

 they were soldered to very thick wires, leading to the bridge. 



In the bridge a galvanometer of the type of du Bots and Ruhfas 

 was used, The magnet, most used was a small half ring magnet of' 

 the newest type with water cooling. For some determinations a 

 large model magnet was used. This new type enables a long series 

 of measurements to be made without introducing an error due to 

 rise of the temperature in the field. 



For the resistance measurements at the temperature of liquid air 

 the newly constructed vacuum cryoarmature on the immersion prin- 

 ciple has been very useful ^). In fact the combination of the strong 

 fields up to 45 K.G. with low temperatures can easily be obtained ; 

 the field was only very slightly diminished by the gaps in the pole 

 pieces in which the Dewar vessel fitted. This strong field is partly 

 due to the use of ferrocobalt for the pole pieces. 



The magnetic field was measured by the usual ballistic method ; 

 nothing particular has to be remarked on this subject. For the esti- 

 mation of the infiuence of the low temperatures on the field between 

 the ferro-cobalt pieces the Dewar vessel with the liquid air was 

 quickly taken away and immediately afterwards the field was measured. 

 This must be done very quickly, as the pole pieces become warm, 

 while also the search coil cools down and at the lower temperature 

 its area is smaller, (and just on this area is based the measurement 

 of the field). Particularly the latter must be avoided as the coefficient 

 of expansion of the material (a hard kind of culophonium) was not 

 known. The coil had been compared previously with a carefully 

 polished glass standard coil.') The influence of the temperature on 

 the field proved to be negligible. 



The current through the magnet, was read on a precision instru- 

 ment of Siemens and Halske. The resistance measurements were 

 carried out at very short intervals. This was desirable to eliminate 



1) H. DU Bois, Ann. der Physik. 42 p. 968, 1913. 



2) See W. J. DE Haas and P. Drapier, Ann. der Physik. 43, p. 673, 1913. 



