256 Professor Fleming [June 5, 



but mucli smaller effect in the case of nickel longitudinally mag- 

 netised. It will be seen that this process of taking the resistance of 

 a conductor in liquid air is one which affords us a very critical means 

 of discrimination as to the chemical purity of a metal. It ranks 

 almost with the spectrosco23e as an analytical method. There is one 

 other method by which we can exhibit the change in conductivity in a 

 metal when cooled, and that is by the increased deflection which a 

 disc of the metal experiences when suspended in an alternating 

 current field in such a position that the plane of the disc is at an 

 angle of 45^ to the direction of the field. 



Time will only permit one brief reference to the behaviour of 

 carbon in regard to electrical conductivity when cooled to low tem- 

 peratures. We have found that carbon in the form of carbon fila- 

 ments taken from various incandescent lamps continued to increase 

 in resistance as it was lowered in temperature. The resistivity at 

 various temperatures of the carbon from an Edison-Swan lamp is as 

 follows : — 



These values, when represented on a chart, give almost a straight 

 line, and show that the resistivity of carbon continually increases 

 as it is cooled, but at a very slow rate. Its temperature coefiicient is 

 therefore negative, and of about the same absolute magnitude as many 

 alloys of high resistivity. The resistivity of this form of carbon is 

 about three thousand times that of silver. Adamantine carbon taken 

 from a Woodhouse and Eawson lamp had ,a resistivity 60 per cent, 

 greater. 



All the so-called insulators — e. g. glass, gutta-percha, ebonite, 

 paraffin — have resistivities enormously greater than that of carbon, 

 but like it, their resistance increases as the temperature is lowered. 

 For the sake of comparison we have placed upon this chart of lines 

 of metallic resistivity (referring to the large diagram used at the 

 lecture) the resistance line of carbon with ordinates drawn 

 to a scale of one-hundredth part of those of the metals. To 

 properly represent to the full scale the line of carbon, this chart, 

 which is 15 feet long, would have to be made one-third of a mile 

 long. If we desired to represent on the same scale the resistivity of 

 gutta-percha, the length of the chart would have to be billions of 

 miles — in fact, so long that light would take 5000 years to traverse 

 it from one end to the other ; even then, to represent to the same 

 scale the resistance lines of paraffin and ebonite, it would have to be 

 thirty or forty times longer.* 



We must next pass on to consider some problems in thermo- 



* The resistivities of platinoid, carbon, and gutta-percha at 0° C. are nearly 

 in the ratio of the numbers 4 x 10^ 4 x 10^ and 4 x 10^3^ 



