1896.] 



on Electric Besearch at Low Temperatures, 



241 



this portion of the work. The final outcome of all failures was the 

 production of a resistance coil of the following form: — Two thick 

 wires of high conductivity copper about 3 or 4 mm. thick are bent 

 as shown in Fig. 1, and wrapped round the lower part with a cylin- 

 drical sheath of thin vulcanised fibre laced to them by a silk thread. 

 On this sheath, which generally had the form of an oval cylinder, a 

 paraffined silk cord was spirally wound so as to leave a helical groove. 

 In this groove was coiled the resistance wire, of known length and 

 section, and its ends were attached by solder to the ends of the thick 

 copper leads. The wire was wound a little loosely in the groove so 

 as to allow for the great contraction which takes place in cooling, 

 and yet the wire was exposed so as to take up instantly the tempera- 

 ture of the bath, whilst at the same time the mass of material to be 

 cooled down was rendered as small as possible. The length of wire 

 employed was generally about one or two metres, and the diameter 

 from about one-twelfth to half 



a millimetre ('003 inch to 

 •02 inch). These mean dia- 

 meters were -measured by 

 the microscope micrometer at 

 about fifty to one hundred 

 places for each metre length 

 of the wire. Having thus 

 prepared a great collec- 

 tion of resistance coils of 

 pure metals and alloys, each 

 in the form of a wire of 

 known length and mean dia- 

 meter, the next operation was 

 the measurement of their re- 

 sistance at definite tempera- 

 tures. For the sake of those not 

 fully familiar with the details 



Coil. 



Resistance, 



Fig. 2. 



Diagram of arrangement of circuits for 

 comparing resistances by means of the 

 differential galvanometer. 



of electrical measurement, a moment's 

 digression may be made to explain two of the principal methods in 

 use. Becquerel's work was chiefly conducted v^ith the difi'erential 

 galvanometer. In this instrument two coils of wire of exactly equal 

 length are coiled on one bobbin, in the centre of which hangs a small 

 magnetic needle. The current from a battery (see Fig. 2) divides 

 at one point, and flows along one path through the conductor or 

 conductors under examination and through one coil (No. 2) of the 

 galvanometer. The other portion of the current flows through a wire 

 of variable length called a rheostat, and through the other coil of the 

 galvanometer, equal in every respect to the first coil, but circulates 

 rpund the needle, N.S., in an opposite direction to that of the current 

 in the first coil. Hence, if the currents are of equal strength the 

 needle is not disturbed at all from its zero position. We can make 

 these currents equal by adjusting the length of the wire of the 

 rheostat so that its resistance is equal to the resistance of the coil 

 Vol. XV. (No. 90.) r 



