602 Hon. R. J. Strutt on the Electrical 



electrodes // lay mainly in the narrow portion dd, and this 

 alone, with the branch a, was kept hot. The limbs hb were 

 immersed in tanks of water to keep them cold, so as to avoid 

 melting the sealing-wax. 



It is evident that if the parts dd had alone been heated, 

 the mercury in them would simply have distilled into the 

 branch a. To avoid this, a was kept somewhat hotter, so 

 that the vapour-pressure of the mercury in it effectually pre- 

 vented the thread of mercury in the parts dd from separating. 

 The resistance between the electrodes //was about J ohm. It 

 was determined by means of a slide-bridge and 1 ohm coil. 

 Though the greater part of the resistance was in the narrow 

 parts dd, the limbs Ob, which were not heated, contributed 

 something. The amount to be allowed could only roughly 

 be guessed, since the temperature of the limbs was uncer- 

 tain, varying from point to point. In fact the experiments 

 made no pretension to accuracy, since the temperature was 

 only estimated without measurement. It was merely 

 attempted to ascertain whether or not the resistance increased 

 enormously. 



Thus, in an experiment, the bridge balanced at 52*4, when 

 the mercury was cold, and at 64*7 when it was heated as hot 

 as a bunsen-burner would make it. This makes the re- 

 sistance 1*7 times as much when hot as when cold. If we 

 make allowance for the fact that the limbs of the tube were 

 not heated, the resistance was probably twice as great at the 

 red heat as at ordinary temperatures. This is something- 

 like what might be anticipated from the known temperature 

 coefficient at lower temperatures. 



The experiments had to be somewhat hastily carried out, 

 since the heat soon travelled down the limbs of the tube by 

 conduction, and made the cement slightly viscous. It then 

 soon yielded to the enormous pressure, and the mercury 

 forced its way out, leaving the top of the tube empty. For 

 this reason it was not possible to go beyond the full red heat 

 attainable in the bunsen flame. The same tube was used 

 to obtain a measurement of the conductivity of saturated 

 mercury vapour at the same temperature. This was easily 

 done. The top branch a of the tube was no longer kept 

 hot. The mercury from the narrow parts dd distilled into 

 it, leaving these parts filled with vapour. The tube was 

 joined up in series with a battery-ceil (E.M.F. 1*5 volts) 

 and a high-resistance D' Arson val galvanometer. A de- 

 flexion of about forty scale-divisions was observed, indicating 

 about 4 x 10~ 7 amperes. The resistance was thus about 





