TRANSACTIONS OF SECTION A. 



559 



li. A Reduction q/ Boivland's Value of the Mechanical Equivalent of Heat 

 to the Paris Hydrogen Scale. Btj Wm. S. Day, Fh.D., Columbia 

 University. 



The measurement of the mechanical equivalent of heat made by Professor 

 Henry A. Rowland at the Johns Hopkins University in Baltimore in 1877-79 ^ 

 is probably the best one that has thus lar been made in which the heat was pro- 

 duced by the expenditure of mechanical energy. Later careful determinations 

 by electrical methods, however, give results higher by about one part in four 

 hundred. The discrepancy may be due to errors in the measurement of energy or 

 in the measurement of temperature. Rowland's measurement of temperature was 

 based on comparisons made between an air thermometer and three Baudin 

 mercurial thermometers, by which he reduced his measurements to the absolute 

 thermo-dynaraic scale. It' was the object of the investigation described here to 

 compare these thermometers with the hydrogen scale of the International Bureau 

 of Weights and Measures at Sevres, near Paris, and make a recalculation of his 

 value of the mechanical equivalent accordingly. 



For this purpose three Tonnelot thermometers, which had been carefully studied 

 at the International Bureau, and compared with their standards at several points 

 of the scale, were obtained and compared with the three principal thermometers 

 used by Rowland in his experiment. These comparisons were made in a horizontal 

 comparison tank, designed and constructed for the purpose. Rowland's thermo- 

 meters were originally compared and used in a vertical position, but the horizontal 

 position was chosen for these comparisons for several reasons of a practical nature. 

 The pressure coefficients of tha thermometers were measured, however, and a 

 pressure correction was applied to each reading. In all other respects the attempt 

 was made to use Rowland's thermometers in the way in which he used them. 



The zeros of the Tonnelot thermometers were determined immediately after 

 each measurement at any given temperature. The ice used in taking the zeros was 

 artificial ice, and was very pure. The thermometers were always read in taking 

 zeros, and in the comparison tank, by means of a reading telescope and micrometer. 



From the comparisons made, corrections were obtained for each of Rowland's 

 thermometers, which, when applied to their indications reduced to the absolute 

 scale by the tables given in his paper on the mechanical equivalent, would make 

 them agree with the Paris hydrogen scale. From these corrections, Rowland's 

 value of the mechanical equivalent was recalculated, taking into account each 

 individual experiment, the thermometers used in it, and the number of observations 

 made with each thermometer. The original values and the corrected values found 

 in this way are compared at several temperatures in the following table. The 

 numbers are in the C.G.S. system and hydrogen scale, and represent the number of 

 ergs required to raise the temperature of one gram of water one degree on the 

 liydrogen scale. 



These values give the same variation for the specific heat of water between 15° 

 and 25° as Griffiths' experiment does ; since if we divide Rowland's corrected values 



' Proc. Am. Acad. (15), 1879, p. 75. 

 " PhU. Trans., 1895, 18GA, p. 458. 



