1896.] 



on Electric Besearch at Loiv Temperatures. 



243 



being joined, as required, in series with each other by plugs, is most 

 commonly used, and it was a most carefully adjusted Elliott bridge of 

 this last pattern which we employed. 



All our resistance measurements have been reduced to express 

 them in terms of the International ohm, as defined by the Board of 

 Trade Committee, and obtained by reference to standard coils care- 

 fully standardised for us at Cambridge. By this means the whole of 

 our wires were measured at five definite temperatures, viz. at about 

 200° C. ; at the temperature of boiling water, 100° C. ; at the tem- 

 perature of melting ice, 0° C. ; at the temperature of solid carbonic 

 acid melting in ether, which gives a temperature of about —78° C. ; 

 and at the temperature of liquid oxygen boiling under a pressure of 

 760 mm.j which gives a temperature of —182° C. 



Fig. 4. 

 Slide wire bridge. Lecture form. 



In this last case the coils were immersed in liquid oxygen con- 

 tained in suitable vacuum-jacketed vessels. In this connection, I 

 should like to express with due emphasis the opinion that none of 

 this low temperature research would have been possible at all with- 

 out the assistance of Professor Dewar's most valuable invention the 

 glass vacuum-jacketed silvered vessel. For much of this work it has 

 been necessary to employ many litres of liquid oxygen and air at a 

 time, and to be able to keep it for hours in a state of perfect qui- 

 escence and absolutely constant temperature, and in no way could 

 this have been done without this beautiful and scientific device. 



Before describing the results of these experiments it may be 

 interesting to exhibit a few of the principal facts. The most strik- 

 ing of them is the very great reduction in electrical resistance, or 

 increase in conductivity, experienced by all the pure metals when 

 cooled in liquid air. Here, for instance, are two coils of iron wire : 

 balancing them on the bridge we find them to be of exactly equal 

 resistance, but if one of the coils is cooled in liquid air its resistance 

 is reduced to about one-tenth of its resistance at the ordinary tem- 



r2 



