HEAT. 



furnished with a thermometer. The bar was raised to a high tem- 

 perature and then placed near the other, which was now cool, so that 

 the surface of the small bar was exposed as nearly as possible to the 

 same conditions, as to loss of heat for the same temperature, as the large 

 bar, and the rate of cooling was observed. Knowing the capacity for 

 heat of the bar and the area of its surface, the rate of cooling at any 

 temperature gave the quantity of heat lost by each unit of area in a 

 given time for that temperature, and thus the quantity of heat lost per 

 second by every part of the larger bar in the first part of the experiment 

 could be determined, and so Q could be calculated. 



Determining the conductivity at different sections of the bar at 

 different temperatures, Forbes was further able to show that the con- 

 ductivity of iron decreases with rise of temperature. 



Tait repeated Forbes's work on the same bar, and used also bars of 

 copper and other metals in the same manner.* Stewart f has also 

 carried out experiments on the conductivities of iron and copper by the 



FIG. 68. Forbes's Bar Experiment. 



method of Forbes, using in place of thermometers a thermo-electric 

 junction inserted in small holes in the bar under observation. 



Lees, in the course of some experiments to be described below, has 

 also used the same method for brass. 



Neumann and Angstrom's Method. The second or temperature- 

 wave method of measuring conductivities was used by Neumann, who 

 heated one end of a bar of the substance experimented on, and then 

 cooled it. Observation of the march of the cooling along the bar enabled 

 him to determine the conductivity. 



Angstrom alternately heated and cooled one end of the bar, and after 

 a time waves of temperature-disturbance travelled regularly along the bar. 

 The rate of progress at different points and the rate of diminution in the 

 amplitude of the disturbance enabled him to calculate the conductivity.! 



Gray's Experiments. A simple mode of determining con- 

 ductivity of metals has been used by J. H. Gray. A wire a few 

 centimetres long, and about 0'2 cm. diameter, was hung vertically with 

 its upper end soldered into the bottom of a copper vessel containing 

 boiling water and its lower end into a copper ball, 5 - 5 cm. in diameter, 

 drilled with a hole in which a thermometer was inserted. The wire was 

 surrounded by cotton wool, and the loss from the sides was negligible. 



* Tait's Heat, chap. xiv. t Phil. Trams.,- 1893, A., p. 569. 



t "Heat," Ency. Brit., 9th ed., or Tail's Heat, chap. xiv. 

 Phil. Tram., A., Part I., 1895, p. 165. 



