ON THE ABSOLUTE EXPANSION OF MERCURY. 11 



inserting a glass plunger, without withdrawing or adding mercury, the levels returned 

 to their previous values within O'Ol mm. in about a minute after the removal of the 

 plunger, in spite of the great length of fine tube through which the mercury had to 

 flow. If, on the other hand, the continuity of the mercury column was broken by a 

 single air bubble in one of the fine tubes, the level could be altered by a centimetre 

 or more on one side without any change taking place on the other. It was feared at 

 first that the presence of air bubbles might be a serious source of error, but the effect 

 produced was so immediately obvious that no uncertainty arose in this way. 



In spite of the care taken in evacuating the apparatus, some bubbles invariably 

 appeared when the apparatus was first heated to high temperatures such as 200 C. 

 to 300 C. after each fresh filling. These bubbles were removed as they appeared by 

 altering the level of the mercury in the gauge, so as to reduce the pressure and drive 

 the bubbles round into the open space where the tap was connected, whence they 

 could be removed by applying the air pump. 



At the highest temperatures, from 200 C. to 300 C., it was found necessary on 

 account of the expansion of the containing tubes, which amounted to nearly 8 mm. 

 at 300 CL, to insert a flexible rubber connection, as indicated in fig. 5b, between the 

 glass taps and the small-bore tubes on the cold side. The T-joint was placed close 

 to the end of the small-bore steel tube on the hot side to facilitate the trapping of 

 bubbles, which were most troublesome at the higher temperatures. Fortunately this 

 trouble tended to disappear as the occluded or dissolved gas was removed by 

 repeated heating of the mercury. 



5. Method of Taking Obsei-vatiotis. 



After starting the water and oil circulations, the heating coils were connected to 

 the electric-light mains. A current of 13 amperes at 100 volts sufficed to raise the 

 temperature about 100 C. in an hour and a half. When the temperature approached 

 the required point, the current was gradually reduced to the value which experience 

 had shown to be sufficient to maintain the desired temperature. The current was 

 then switched over to a large battery of accumulators, which made it possible to 

 keep the temperature very nearly constant with slight occasional adjustments. 

 But as the cold columns rose very slowly in temperature, at the rate of about a tenth 

 of a degree in half an hour, the current was generally set to give a slightly greater 

 rate of rise for the hot column during half an hour or so, followed by a slightly 

 slower rate for another half hour, so that during the first period the difference of 

 temperature and the difference of level might be slowly increasing, and during the 

 second period slowly diminishing, at nearly the same rate. By taking observations in 

 this way the effects of lag, if any, would be eliminated from the results. The actual 

 readings of temperature and of difference of level were plotted on a large scale 

 (10 cm. to 4 cm. on the bridge wire and 10 cm. to 2 mm. of the difference of level), 



c 2 



