426 Profs. J. Dewar and J. A. Fleming. On ike 



observations, and completed them, as far as possible, by making- 

 measurements of the electrical resistance of a wire of pure bismuth, 

 placed transversely to the direction of the field of an electromagnet, 

 and at the same time subjected to the low temperature obtained by 

 the use of liquid air. 



Sir David Salomons was so kind as to lend us for some time his 

 large electromagnet, which, in addition to giving a powerful field, is 

 provided with the means of easily altering the interpolar distance of 

 the pole pieces, and also for changing from one form of pole piece to 

 another. 



The form of the pole piece most frequently used was that of a 

 truncated cone. The magnet was always excited by a constant 

 current obtained from a constant potential circuit. To save the 

 considerable labour of determining again and again the strength of 

 the interpolar field, this was determined once for all, corresponding 

 to various interpolar distances and a given exciting current. The 

 field was measured by suddenly removing from it a small exploring 

 coil of wire of known area, the same being connected to a standardised 

 ballistic galvanometer. 



By this means a curve was constructed which showed at once the 

 axial interpolar field at the central point in terms of the interpolar 

 distances, the magnetising current being kept constant. This curve 

 proved, as was to be expected, to be nearly a rectangular hyperbola. 



This being done the bismuth wire to be examined was formed into 

 a narrow loop of a single turn, about 3 or 4 cm. in length, and the 

 ends soldered to leading-in wires of copper. The loop was placed in 

 a small glass vacuum vessel, with the plane of the loop perpendicular 

 to the direction of the axial magnetic field of the magnet. The loop 

 was placed at equal distances from the two pole pieces, and in a 

 nearly uniform field of known strength. 



The vacuum vessel was then filled up with either liquid air, a 

 solution of solid carbonic acid in ether, or else simply with paraffin 

 oil. In a fourth case the vacuum vessel was closed, and liquid air 

 having been placed in it, this liquid was caused to boil under a 

 reduced pressure of 25 mm., thus giving a temperature falling as low 

 as 203 C. In another experiment the vacuum vessel was dispensed 

 with, the bismuth wire was simply wrapped in cotton wool, placed 

 between two pieces of thin mica between the pole pieces, and by 

 pouring upon the wrapping a copious libation of liquid air, the 

 temperature of the bismuth wire was reduced to 185 C. 



In all cases great care was taken to avoid thermo-electric complica- 

 tions, by providing that the soldered junctions by which the bismuth 

 wire is connected to the copper leading-in wire were at exactly the 

 'same temperature, and to secure this the junctions were always kept 

 well covered with the refrigerating solution. 



