4 BULLETIlSr 420; U. S. DEPARTMENT OF AGEICULTUEE. 



of Standards. A Weston cell certified by that bureau furnislied the 

 known potential. A switch, essentially the same as that described 

 by White/ was used to correct for parasitic electromotive forces. 



In the set-up the hot junction of each element was placed where 

 desired and the cold one packed in crushed ice within a thermos 

 bottle. The terminals were led to mercury pools in a block of par- 

 affined wood- protected from air currents. Connection with the 

 potentiometer was made through copper leads whose bared and amal- 

 gamated ends could be dipped in the successive pairs' of mercury 

 pools. In this way any element could be quickly and easily brought 

 into circuit. 



Differences of one microvolt could be detected by the system. This 

 corresponds to approximately 0.018° F. (0.01° C). The experimental 

 errors incident to rapid readings resulted in an accuracy of about 

 0.09° F. (0.05° C). 



In determining the difference in temperature between the top and 

 bottom of a bottle a differential thermo element was used ; this con- 

 sisted of two pairs of copper-constantin couples in a thin glass tube. 

 The hot and cold junctions were placed so that one would be about 

 one-half inch above the bottom and the other just beneath the 

 surface of the liquid when cool. This differential couple was not 

 cahbrated, and its design was not all that could be desired, but it 

 doubtless was sufficiently accurate to furnish a fairly good measure 

 of the differences in temperature between the top and bottom layers. 

 In this connection it may be noted that the thermometers used in this 

 work are numbered from 1 to 10, which numbers are used to identify 

 them with the curves in the drawings showing the results of the various 

 experiments. 



Air measurements were taken with a Taylor-Pitot tube placed in 

 the center of the straight part of the discharge pipe from the box, 

 about 4 feet from the nearest bend. The tube was arranged to give 

 both the impact and static pressure, their difference, or the velocity 

 head, being indicated on a graduated water column. Only one tube, 

 situated in the center of the pipe, was used. The average velocity 

 head across the entire section of the pipe was determined by multi- 

 plying this center reading by the factor 0.91. 

 ■ The velocities obtained with the Pitot tube were checked with an 

 anemometer which had been calibrated by the Bureau of Standards. 

 The anemometer was moved across the end of the outlet pipe in such 

 a way as to give what was considered an average reading. As the 

 temperature of the air in the outlet pipe from the box was, of course, 

 higher than that of the incoming air, the volumes were corrected in 

 the ratio of their absolute temperatures to give the inlet volume. 



1 White, Walter P. Journal of American Chemical Society, 36, p. 1856, 1914. 



