Gold for Weak Magnetic Fields. 73 



metals in very weak magnetic fields, to see if there existed, 

 in that region, any change similar to the one quoted above 

 for bismuth-tin alloys. 



The method employed in this investigation was a slight 

 modification of that given by Dr. A. Lebret*. It is repre- 

 sented diagrammatically in fig. 1. The current from a 



Fief. 1. 





^\\p 







i ;h,' 



i , 



1 i 

 i i 



V 





\) 





,d= 



U-J 





storage-cell B passed in turn through the reversing key K; 

 a small resistance S (about 0'02 ohm), and through the plate 

 of metal to be experimented upon, shown at A. This current 

 is spoken of below as the primary current. From the ends 

 of the small resistance S a shunt current was led through 

 the resistance-box K, and then through one set of coils (G^) 

 of the differential galvanometer. This circuit is referred to 

 below as the shunt circuit. The current from the " Hall 

 electrodes M Hj and H 2 was sent through the second set of coils 

 (G 2 ) of the galvanometer, and is called the secondary current. 

 In order to prevent changes in the strength of the magnetic 

 field from acting inductively on that part of the secondary 

 circuit that lay between the magnet poles, the device of 

 S. Bidwellf was adopted. This consists in branching part 

 of the circuit as shown at H 2 P. Now as a change in the 

 magnetic field acts in opposite senses on the partial circuits 

 Hj H 2 gx P and Hj H 2 q 2 P, these effects may be made to 

 neutralize one another simply by bending one of the wires 

 (<2i or q 2 ) so as to equalize the magnetic flux through the two 



* Dr. A. Lebret : " A New Method of Measuring the Hall Effect, 

 especially with reference to Variation of Temperature." Communica- 

 tions from the Physical Laboratory of the University of Leiden, No. 19. 

 (Translated from Verslarjen en Medeelingen van die Kon. Acad, van 

 Wetenschappcn van 18 April, 1895, p. 284.) 



f Phil. Mag. vol. xvii. (April 1884) ; see also Lebret (he. cit.). 



