On the Thermal Conductivity of Water. 47 



false bottom to B), which is kept at a uniform temperature 

 by cold water flowing through it. 



In order that the whole of the heat may flow downwards 

 good thermal insulation is provided above D in the shape of 

 cotton-wool, W, and a second flat coil E (the " balancing- 

 coil ") is provided, through which a current may be sent to 

 prevent a temperature-difference between D and E ; the two 

 junctions of a thermopile, soldered respectively to metal disks 

 at J and F, being used for obtaining this condition. Definite 

 and fairly good thermal contact between D and the bottom 

 of A is further obtained by imbedding Din a layer of molten 

 paraffin wax about 0*5 centim. thick, D being held against 

 the bottom of A until the wax solidifies. 



Although near the circumference of D the flow of heat has 

 a horizontal component, it is linear near the centre, and it is 

 to this region that the following applies. 



Take 6 to be the difference of temperature between the 

 centre of the heating-coil and the sink of heat, and x the 

 thickness of a layer of water 1 square centim. in area, and of 

 thermal conductance equal to that per square centim. between 

 coil and sink. Then if C is the current through the heating- 

 coil, R its resistance, and A its area, 



0-2406 C 2 ^=ki 

 A x 



where k is the thermal conductivity of water. 



Now increase the thickness of the water layer by y, 



0-2406 2 ? = £ - 01 



A ' x+y' 

 whence k is given at once in terms of the measured quantities 



*=°" 24oiiC2 !.^ (D 



The method in this simple form is, however, open to certain 

 objections. In the first place, the strength of the current in 

 D and the temperature and rate of flow of the water in 

 the sink must be kept constant over long periods ; and as even 

 small changes in their values occurring in the middle of an 

 experiment may easily produce changes in 6 exceeding that 

 produced by the change in thickness of the water layer, this 

 is not an easy condition to fulfil. To avoid this difficulty we 

 modified the apparatus to work differentially. A second pair 

 of vessels A', B' were constructed exactly similar to A and B 

 (fig. 1) ; the same currents of electricity and water were sent 



