﻿2 MORRIS-AlREY & SPENCER, Resistivity of Carbon. 



of contact with their neighbours, and increasing the 

 resistance of the mass as a whole. 



The great variation of the resistance of carbon in its 

 different structures have led to the neglect of this element 

 except in the case of high temperatures where a know- 

 ledge of its resistance was of commercial value. It 

 seemed, however, of interest to examine some particular 

 variety of carbon, and trace its variation, with temperature, 

 to determine whether its resistance still increased at the 

 same rate as the temperature became lower, or whether it 

 finally behaved like a metal. 



The conductors selected for the test were filaments * 

 taken from Ediswan incandescent lamps ; they were about 

 iocms. long, and had a diameter of cross section of about 

 •00355 cms., and a resistance of 1,000 ohms at ordinary 

 temperatures. The specific volume resistivity (i.e., the 

 resistance between opposite faces of a cube of the material 

 having 1 cm. edge) was calculated from determinations of 

 resistance on the assumption that the dimensions did not 

 alter with the temperature, sufficiently to produce an effect 

 comparable with that observed. The problem was then 

 to measure the resistance of a filament of carbon of known 

 dimensions, maintained at various low temperatures and 

 to determine those temperatures. The latter measure- 

 ments were made by observing the resistance of a 

 platinum thermometer which had been previously 

 standardised. This consisted of a metre of pure platinum 

 wire, diameter -~<j mm. and resistance 20 ohms, wrapped 

 in the form of a spiral on a small rod of some insulating 

 material. The carbon loop was soldered by means of its 

 platinum terminals to two stout copper wires, and the 



* These are made by carbonising the precipitate obtained on passing a 

 fine jet of a solution of cotton in zinc chloride into alcohol. The pores are 

 filled up by "flashing" the filament in benzine vapour and platinum 

 terminals attached by depositing carbon on the junctions, 



