20 



PHYSICAL LABORATOKY: COMPARISONS OF STANDARDS 



[Johns Hopkins University Circulars, N~o. 3, p. 31, 1880] 



In order to secure uniformity throughout the country in certain 

 physical standards, and to facilitate the use of the absolute system of 

 heat measurement, it has been thought advisable to organize in the 

 physical department of this University a sub-department, where com- 

 parisons of standards can be made. 



Comparison of Thermometers. At present we are only able to make 

 comparisons of thermometers, and so to reduce their degrees to the abso- 

 lute scale of the perfect gas thermometer. 



As the work is very laborious, it is proposed to make this sub-depart- 

 ment self-supporting, by a system of fees sufficient to cover the bare cost 

 of the labor, so that all may avail themselves of the facilities here 

 offered. 



In a recent study of standard thermometers by Geissler, Baudin, 

 Fastre, Casella and from Kew, and the comparison of the same with 

 the air thermometer, the differences due to the variety of the glass 

 amounted to 0-2 or 0-3 C., and the differences from the air thermom- 

 eter were as high sometimes as 0-3 C. at the 40 point. 



The error from using uncompared mercurial thermometers in calori- 

 metric investigations may amount to one or two per cent. For this 

 reason the air thermometer has been taken as the standard, and all com- 

 parisons will be reduced to the final absolute standard of the perfect 

 gas thermometer. 



Very complete studies of thermometers have been made between 

 and 40 C., and a less complete study between and 100, and be- 

 tween 100 and 250. Up to 100 our thermometers have not only been 

 compared with the air thermometer, but also with standards by Fastre, 

 Geissler, Casella, Baudin and from Kew. 



The study from to 40 has been published by the American Acad- 

 emy of Sciences, at Boston, in a memoir on the Mechanical Equivalent 

 of Heat. One of our thermometers is also now in the hands of Dr. 

 Joule, who has compared it with the original thermometers used by him 

 in the determination of the Mechanical Equivalent of Heat. 



