Ox THE MECHANICAL EQUIVALENT OF HEAT 351 



zero returned from 0-170 to 0-148 in two hours and a half. 

 After heating to 100, the zero returned from 0-347 to 0-110 

 in nine days, and to 0-022 in one month. Eeasoning from this, I 

 should say that in one week thermometers which had not been heated 

 ahove 40 should be ready for use again, the error being then supposed 

 to be less than 1 in 4000, and this would be partially eliminated by 

 comparing with the air thermometer at the same intervals as the ther- 

 mometer is used, or at least heating to 40 one week before comparing 

 with the air thermometer. 



As stated before, when a thermometer is heated to a very high 

 point, its zero point is raised instead of lowered, and it seems probable 

 that at some higher point the direction of change is reversed again; 

 for, after the instrument comes from the maker, the zero point con- 

 stantly rises until it may be 0-6 above the mark on the tube. This 

 gradual change is of no importance in my experiments, as I only use 

 differences of temperature, and also as it was almost inappreciable in 

 my thermometers. 



Another source of error in thermometers is that due to the pressure 

 on the bulb. In determining the freezing point, large errors may be 

 made, amounting to several hundredths of a degree, by the pressure of 

 pieces of ice. In my experiments, the zero point was determined in 

 ice, and then the thermometer was immersed in the water of the com- 

 parator at a depth of about 60 cm. The pressure of this water affected 

 the thermometer to the extent of about 0-01, and a correction was 

 accordingly made. As differences of temperature were only needed, 

 no correction was made for variation in pressure of the air. 



It does not seem to me well to use thermometers with too small a 

 stem, as I have no doubt that they are subject to much greater irregu- 

 larities than those with a coarse bore. For the capillary action always 

 exerts a pressure on the bulb. Hence, when the mercury rises, the 

 pressure is due to a rising meniscus which causes greater pressure than 

 the falling meniscus. Hence, an apparent friction of the mercurial 

 column. Also, the capillary constant of mercury seems to depend on 

 the electric potential of its surface, which may not be constant, and 

 would thus cause an irregularity. 



My own thermometers did not show any apparent action of this kind, 

 but Pfaimdler and Platter mention such an action, though they give 

 another reason for it. 



