AND LIQUIDS AND THEIR VARIATION WITH TEMPERATURE. 421 



and 371 ceutims. internal diameter, which in turn was supported by a disc of nickel- 

 plated copper, L, 7 centims. diameter and '3 centim. thick, to which it was cemented 

 by rubber cement. 



The copper discs will be called in order downwards, the " cover," " upper," " middle," 

 and " lower " copper discs, respectively. 



The discs were placed with the flat surfaces horizontal,"' in a flat cylindrical air 

 bath, fig. 12, p. 419, consisting of an upper and a lower half, each capable of being 

 heated by means of an insulated coil of platinoid wire wound round it, and the 

 lower half provided with a compartment through which a stream of cold water 

 could be sent to cool the bath rapidly. During a test of thermal conductivity, this 

 compartment contained air only. 



A thermo-j unction was soldered inside the top of the upper part of the enclosure, 

 and it is assumed that the air to which the top and side surfaces of the discs lose 

 heat, has the temperature given by this thermo-junction. The thermo-junctions in 

 the upper discs were those described (p. 419), and were arranged vertically under 

 each other. In order to bring the junction in the lower disc vertically under those in 

 the upper, the holes in the sides of the lower disc were made correspondingly deeper, 

 and for a depth of 1'5 centims. wider than those in the upper discs, and were lined 

 with thin glass tubes to prevent the wires making contact with the disc at any 

 other points than the junctions. 



The liquid to be tested was placed in the ebonite ring, and had therefore the same 



thickness as the ebonite. 



* 



The heat flowing through the under surface of the middle copper disc was con- 

 ducted away partly by the liquid under test, partly by the ebonite. The amount 

 conducted away by the ebonite could be calculated if the shape of the stream lines 

 and the thermal conductivity of the ebonite were known. It is, however, much more 

 accurate to determine this quantity by a separate experiment, substituting air 

 for the liquid and so arranging the amount of heat supplied, that the temperature 

 difference between the middle and lower discs is the same as in the experiment on 

 the liquid. The heat conducted through the air is then small, and can be calculated 

 from the known thermal conductivity of air.t The remainder of the heat flowing- 

 through the under surface of the middle disc is conducted through the ebonite to 

 the lower disc, and is the quantity required. 



Theory of Ebonite Ramg Apparatus. 



If r = radius of copper and glass discs, 

 t a = thickness of glass disc, 



* The disc of liquid being thin, convection currents only come in to a slight extent when the discs 

 are not qnite horizontal. In the case of a disc of water, an inclination of 15 to the horizontal only 

 increased the apparent conductivity 1 per cent. 



t WINKELMANN'S value has been nsed, ' Wied. Ann.,' vol. 48, p. 186, 1895. 



