240 



Professor Fleming 



[June 5, 



resistances of an iron wire and a salt solution which were marvellously 

 accurate, when we consider that his only means of measurement was the 

 comparison of electric shocks taken through the bodies to be examined. 

 Not until after the invention of the battery and galvanometer was 

 it clearly proved that differences exist between the conducting powers 

 of metals; but by Davy, Becquerel, Ohm, Pouillet, Fechner and 

 others all the fundamental facts were ascertained, and the classical 

 researches of Wheatstone and later of Matthiessen gave us the accurate 

 lavrs and constants of electrical conduction. By these investigations 

 it was shown that in the case of electric conduction through metallic 

 wires of uniform sectional area their total resistance was proportional 

 to the length, inversely as the cross section, and also proportional to 

 a specific constant for each material called its resistivity. Moreover, 

 it was found that this resistivity was considerably affected by tem- 

 perature, generally being increased in metals by rise of temperature, 



and decreased for carbon, electro- 

 lytic liquids and many badly con- 

 ducting bodies. 



Although much knowledge of 

 the behaviour of pure metals and 

 alloys in regard to electric con- 

 duction has thus been accumu- 

 lated, we considered that it would 

 be of great scientific interest to 



examine with care the changes 

 occurring in the conductivity of 

 these bodies, or reciprocally in 

 their resistivity, when cooled to 

 temperatures of two hundred 

 degrees or more below the Cen- 

 tigrade zero by the aid of liquid 

 oxygen and liquid air.* Knowing the great influence of very 

 small quantities of impurity on this quality, our first attention was 

 directed to obtaining samples of alloys and metals in a state of great 

 chemical purity, in giving to wires drawn from them a suitable form, 

 and in devising a convenient support or holder by which the electrical 

 resistance of the wire might be measured when immersed in liquid 

 oxygen or liquid air, either in quiet ebullition in an open vessel, or 

 under reduced pressure in a closed one. It will be unnecessary to 

 dwell on the difficulties surrounding the preparation of these 

 accurately drawn metallic wires of pure metal. Suffice it to say 

 that our obligations to Mr. George Matthey, Mr. Edward Matthey, 

 Mr. J. W. Swan and other friends were very great with respect to 



* Almost the only experimental work previously clone in this subject seems 

 to have been that of Cailletet and Bouty (' Journal de Physique,' July 1885), on 

 the ' Kesistance of Metals at - 100° C.,' using ethylene as a refrigerating agent ; 

 and a research by Wroblewski, on the 'Resistance of Copper at very Low 

 Temperatures' ('Comptes Rendus,' 1885, vol. ci. p. IGl). 



Fig. 1. 

 Low temperature resistance coil. 



