CONDUCTIVITY. 



99 



A thermometer in the boiler gave the temperature of the upper end of 

 the wire, and that in the ball gave practically the temperature of the lower 

 end. The mean slope of temperature along the wire was thus known. 

 The quantity of heat conducted per second was determined by the rate of 

 rise in temperature of the copper ball. Assuming that the conductivity 

 varies uniformly with the temperature, the experiments gave the con- 

 ductivity at the mean temperature of the wire, about 53. The value of 

 the method lies in its applicability to metals which cannot easily be 

 obtained in large masses. 



Berget's Experiments on Mercury. Berget* determined the 



conductivity of mercury, using a column of the liquid in a vertical glass 

 tube surrounded by a much wider column of the same liquid, both being 

 heated at the top by steam or mercury vapour, and conducting the heat 

 downwards to a base kept at 0. The central column terminated in an 

 ice calorimeter, the central column and the wider surrounding column 

 being at the same temperature at the same level. The heat flow was 

 vertically downwards, a conclusion confirmed by the temperature-slope, 

 which was uniform down the column. The quantity of heat arriving 

 at the lower end was determined by the amount of ice melted. The 

 temperature at various points along the column was determined by 

 thermo-electric junctions, and thus the temperature slope was known. He 

 found that the conductivity was constant between and 100, and that 

 it then diminished. 



The following table gives some of the results obtained for metals, 

 the different values for the same metal by the same observer being 

 obtained with different specimens. 



Journal de Physique, viii., 1888, p. 503. 



