TABLES 21O-213. 

 MECHANICAL EQUIVALENT OF HEAT. 



TABLE 210. Summary of Older Work. 



197 



Taken from J. S. Ames, L'equivalent mecanique de la chaleur, Rapports presentes au congres 



international du physique, Paris, 1900. 

 Reduced to Gram-calorie at 20 C. (Nitrogen thermometer). 



* Admitting an error of i part per 1000 in the electrical scale. 



The mean of the last four then gives 



1 gram (20 0) calorie = 4.181 X 10 7 ergs. See next table, 

 x gram (15 C.) calorie = 4.185 X io 7 ergs assuming sp. ht. of water at 20 = 0.9990. 



TABLE 211. (1915.) Best Value, Electrical and Mechanical Equivalents of Heat. 



Since the preparation of Dr. Ames' Paris report, considerable work has been done on the me- 

 chanical equivalent of heat, including recomputations from the older measurements using better 

 values for some of the electrical relations, etc. Taking all the available material into account the 

 U.S. Bureau of Standards has adopted, provisionally, the relation 



1 (20 C.) gram-calorie = 4.183 international electric Joules. 



No exact comparison between the results of electrical equivalent and mechanical equivalent of 

 heat measurements can be made without exact knowledge of the relations between the interna- 

 tional and absolute electrical units. A recent absolute measurement of absolute resistance by F. 

 E. Smith of the National Physical Laboratory of England indicates a difference of one part in 2000 

 between the international and absolute ohms. Pending the general acceptance of some definite 

 figure for this relation it is useless to fix upon a single value to use for " J " better than about one 

 part in a thousand. The value 



4-183 international joules = probably 4.184 mechanical loules. 

 This value is made the basis of the following, table. 



TABLE 212. Conversion Factors for Units of Work. 



The value used for g is the standard value, 980.665 cm. per sec. per sec. =32-174 feet per sec. per sec. 

 *The values thus marked vary directly with " g." 

 tThe values thus marked vary inversely with " g." For values of g see Tables 565-567. 



TABLE 213. Value of the English and American Horsepower (746 watts) in Local Foot-pounds 

 and Kilogram-meters per Second at Various Altitudes and Latitudes. 



SMITHSONIAN TABLES. 



