Resistivity of Mercury at the Temperature of Liquid 



Air. 77 



after being cleaned, was then very carefully filled with the purified 

 mercury, and by running the mercury through a spiral several times, 

 all air bubbles and air film were finally removed. Into the wider 

 ends of the spiral, amalgamated copper electrodes were introduced, 

 consisting of copper wire 4'4 mm. in diameter ; the wider terminal 

 ends of the spiral were then closed by paraffined corks to keep the 

 copper electrodes in position. This spiral, full of mercury, was 

 placed in a test-tube, and paraffin wax cast round it so as to enclose 

 it entirely, leaving only the copper electrodes protruding. In order 

 to determine the temperature of the mercury in the glass spiral tube, 

 a platinum wire, the resistance of which was known at all tempera- 

 tures down to the temperature of liquid air, was also embedded in the 

 paraffin wax closely in contact with the glass spiral, and proper 

 electrodes brought out to enable the resistance of this platinum wire 

 to be determined. This mass of paraffin wax was then cooled down 

 in a vacuum vessel kept filled up with liquid air until the whole mass 

 reached the temperature of the liquid air. The glass spiral and 

 thermometer enclosed in wax was then removed from the bath of 

 liquid air and placed in a vacuum-jacketed test-tube, in order that it 

 might warm up with extreme slowness to the ordinary temperature 

 of the air. 



Having in this manner cooled the mass of paraffin enclosing the 

 glass spiral filled with mercury and the platinum resistance wire 

 entirely to the temperature of liquid air, a series of observations were 

 taken with the aid of two observers, one measuring the resistance of 

 the mercury by a Wheatstone's Bridge, while at the same time the 

 other observer at another slide wire bridge measured the resistance 

 of the platinum wire, these observations being taken quite simul- 

 taneously, and continued whilst the mass heated up from 197'9 

 (platinum temperature) to 0. All proper corrections were then 

 applied to correct for the resistance of the connecting wires and the 

 bridge temperature ; and the observed resistance of the platinum 

 wire employed was corrected to determine from its resistance tem- 

 peratures in terms of the standard platinum thermometer employed 

 by us in our investigations on the thermo-electric power of metals 

 and alloys (see Dewar and Fleming, 'Phil. IVIag.,' July, 1895, p. 95). 

 This standard thermometer has always been denoted by the letter P x . 

 The following table shows the corrected resistance of the mercury 

 column and the corresponding platinum temperatures, as also the 

 specific resistance of the mercury calculated from the accepted re- 

 sistivity at C. : 



