Gold for Weak Magnetic Fields. 77 



The resistance of either set of four coils was a half an ohm. 

 With a period of 15 seconds it gave a deflexion of 1 mm. 

 scale-division at a little over a metre's distance, for a difference 

 of current of 10 — 9 ampere. The ratio of the galvanometer 



constants of the coils — was 0*96. 



Leads from the secondary electrodes of the plate were 

 soldered to the terminals of one set of coils of the galva- 

 nometer, so that all joints in the secondary circuit were 

 soldered. Each joint was well wrapped in cotton-wool to 

 prevent rapid changes of temperature from draughts (fee. 

 All wiring was tested as it was set up to ensure that there 

 was no effect on the galvanometer from the primary or 

 magnetizing currents. The entire secondary circuit, inclu- 

 ding the galvanometer, was set on paraffin. 



As it is not practically possible to solder the secondary 

 electrodes to points that shall be the ends of one and the 

 same equipotential line when the primary current is passing 

 through the plate, there being no magnetic field through the 

 gold, the first operation was to cut notches in the top or 

 bottom edges of the foil (as described in Hall's original paper, 

 loe. cit.), to bring the potential at the upper electrode to, as 

 nearly as possible, the same value as that at the lower elec- 

 trode. This adjustment could never be carried out exactly. 

 The current passing through the circuit due to this was always 

 somewhat greater than that due to the effect sought. To 

 overcome this difficulty Lebret {loc. cit.) used a u by-current," 

 shown in fig. 1 as the dotted connexion between K and H 2 . 

 By adjusting the value of the resistance R ; the potential at 

 H 2 could be brought exactly to that of H^ It seemed to me 

 that this w r as more open to objection than the method of 

 omitting the *' by-current " entirely, and measuring merely 

 the change of the current in the secondary due to the assisting 

 or opposing transverse electromotive force in the plate. Ac- 

 cordingly, after H^ and H 2 were brought to be as nearly as 

 possible at the same potential (when the primary current was 

 flowing), and after the divided part of the secondary circuit 

 had been adjusted so that even a reversal of the magnetic 

 field left no resultant current through the galvanometer, — the 

 order of operations was as follows : — 



(a) Measurement of the magnetic field by reversing it through 

 the coil carried in front of the gold plate, and comparing 

 the throw obtained in the ballistic galvanometer with that 

 due to the quick turning of an earth-inductor* through 1 80°. 



* The earth-inductor was standardized in place by comparison with 

 the induced current obtained by reversing a knows current through a 

 aet of coils of known dimensions. 



