in a Magnetic Field at Different Temperatures. 691 



This small armature was connected in series with an 

 exactly similar but larger one revolving about a horizontal 

 axis at the centre of a horizontal standard solenoid. The 

 armatures were driven from a countershaft to which were 

 attached pulleys of the requisite diameter, the countershaft 

 itself being driven by a small motor. 



The currents generated in the armatures were arranged to 

 neutralize one another in a circuit in which the sensitive 

 galvanometer used throughout these experiments was included, 

 and by adjusting the field of the solenoid a complete balance 

 was obtained between the armature currents. It was then 

 found that the current in the solenoid necessary to produce 

 this neutralization of those in the armature was quite inde- 

 pendent of the temperature of the bath surrounding the poles 

 of the magnet. 



This method of measuring fields was therefore abandoned 

 in favour of the practically simpler one of snatching a 

 small exploring coil from the space between the pole- 

 pieces. 



This latter operation was effected mechanically by attach- 

 ing the exploring coil to one end of a lever and fixing the 

 coil (by means of a thread attached to the other end of 

 the lever and passing over a pulley) between the pole- 

 pieces against the elasticity of a rubber cord. On burning 

 the thread the contraction of the rubber cord effectively 

 removed the coil from the field. 



Sr 

 It will be noticed that the values of — at the temperature 



r L 



of steam are only about J of those obtained at ordinary 

 atmospheric temperatures, a fact which gave some reason to 

 hope that the changes occurring at the temperature of liquid 

 air might be measurable in spite of the much smaller fields 

 that must necessarily be employed in this case and of the 

 difficulty of preserving these temperatures to any extra- 

 ordinary degree of constancy. 



Accordingly, the experiments were tried with the apparatus 

 already described, which allowed the pole-pieces to be brought 

 within 1*8 cm. of one another. In working with liquid air the 

 difficulty of obtaining a constant temperature and therefore 

 a constant resistance was very pronounced, the resistance 

 always rising during an experiment — in the course of experi- 

 ments tabulated below from r = 4'12 to r=4*20 — and the 

 accuracy of the results suffers accordingly. 



There can be no doubt, however, that these results are 

 quite of the right order. A characteristic set of readings 

 obtained is given below, 



2 Y 2 



