309 
The general character of the isotherms is also conserved at hydrogen 
temperatures; the field at which the resistance begins to increase 
practically proportionally to the field itself is about 12000 gauss just 
as at liquid air temperatures. The gradual transformation from the 
change at small fields to the practically linear change in strong fields 
takes place in the same way at each temperature. 
§ 3. The Harr-efjeet and the increase of resistance for plates of 
compressed electrolytic bismuth. Experimental method. 
The method adopted was that developed and applied by Lesret and 
VAN EVERDINGEN in their dissertations (see Suppl. N°. 2); in it all 
disturbing influences are eliminated. A diagram is given in Plate 3 
of the Supplement quoted, and for all matters concerning the arran- 
gements for measuring we may refer to Chapter I of that paper. 
Circular plates were used to which were soldered with Woop’s alloy 
the primary and Harr electrodes as well as two auxiliary electrodes 
(placed on the diameter in the direction of the main current). All 
were point electrodes '). 
Choosing our notation to correspond with that of the Supplement 
quoted let us write e for the potential difference between the Harr 
electrodes, / for the main current and d for the thickness of the 
plate. The Hat constant A is given by 
ed 
SS 
Let us also write A, for the resistance of the secondary circuit outside 
the plate, r for the resistance of the shunt of the compensating 
circuit, g for a constant determined by the differential SON 
employed, and fy for the resistance determined by 
Bat 1 1 
Rid Bi 
in which Ry and Ap are magnitudes obtained from the resistances 
of the compensating circuit with reversal of the main current when 
the field commutator stands in each experiment in the positions A 
and B respectively; we then obtain 
R= rdq Be 
ARG 
The change in the resistance was also measured as well as the 
Hat effect. 
At ordinary temperature the bismuth plates showed no asymmetry 
1) Van EvERDINGEN has solved the problem theoretically for point electrodes with 
circular plates. 
