the Thermal Conductivity of Water. 51 



Containing Vessels and their Fittings. 



The two pairs of containing vessels (A, B, fig. 1) are sup- 

 ported inside a thick cast-iron box, A and A' being let down 

 into the box through holes in its upper surface, their weight 

 being in each case supported by three levelling-screws resting 

 on the top of the box. The bottoms of A and A' can thus be 

 levelled while their positions relatively to the cast-iron box ■ 

 are fixed so long as the levelling-screws rest upon it. The 

 lower halves B and W, on the other hand, are each supported 

 within the box by a separate system of levers in such a way 

 that by turning micrometer-screws either B or W may be 

 caused to move vertically and parallel to itself through a 

 known distance. By this motion the thickness of the water 

 layer is altered. The following numbers were determined 

 directly by a reading-microscope. 



Change in thickness of water layer per turn of 

 micrometer-screw. 



B (R.H.) . . 0-04100 cm. 

 B' (L.H.) . . 004090 „ 



The vessels B, W are of copper, the upper portions forming 

 open dishes with thick bottoms, and being continued below as 

 the hollow boxes C. Through C runs a copper tube H which 

 opens through the middle of the upper dish by an orifice of 

 about 0'5 mm. diam., and is for the purpose of introducing 

 the liquid whose conductivity is to be measured between A 

 and B. By this plan air-bubbles are swept away from the 

 centre of the liquid layer if accidentally present. G is the 

 waste-pipe for emptying B. Under C is fixed a thermopile- 

 junction K of german-silver and iron wires soldered to a 

 metal disk, and electrically insulated from C, but in good 

 thermal contact with it. 



With regard to the flow of water through the sinks 0, C', 

 it was found that even with the differential method of working 

 fluctuations in the flow were troublesome because they reached 

 and told upon one sink before the other. We therefore fitted 

 up a small cistern in the laboratory which was fed through a 

 ball-tap from the mains and supplied the sinks only. The 

 entrance of bubbles to C and C/ had a very serious effect on 

 the heat-flow, and was prevented, by a trap. 



The upper vessels A, A x . containing the heating-coils gave 

 a great deal of trouble. Metal bottoms we found useless on 

 account of the large horizontal component they introduced 

 into the heat-flow. Glass bottoms, on the other hand, possess 



E 2 



