ON THE SPECIFIC HEAT OF WATER. 211 



less than 30 seconds between 7th January and 9th August, 1910, or less than 

 1 part in 70,000. The correction was therefore inappreciable. 



Mass. The weights used were kindly calibrated under the direction of Major 

 MAOMAHON, F.R.S., at the Standards Department of the Board of Trade. The 

 errors were of the order of 3 or 4 parts in 1,000,000. All weighings were corrected 

 for air displacement. 



Temperature. Four mercury thermometers, each having a range of 50 C., were 

 used for determining the temperature of the contents of the calorimeter, and for 

 calibrating the other thermometers used. These four standards were specially 

 calibrated at the National Physical Lalx>ratory, and corrections (to the nearest 

 0'005 C.) were given for every five degrees to reduce the readings to the hydrogen- 

 thermometer scale. Two of these thermometers had been previously calibrated in 

 1903 at the lieichsanstalt at Charlottenburg. The two sets of calibrations did not 

 differ, except at one point, by more than 0'01 C. All these four standards bad 

 freezing-points marked, which were checked from time to time. The two chief 

 standards (from C. to 50 C. and from 50 C. to 100 C.) experienced a change of 

 only 0'01 C. in the freezing-point between the years 1903 and 1910. The two 

 other standards (from 25 C. to 75 C. and from 75 C. to 125 C.) were calibrated at 

 the National Physical Laboratory in July, 1910, and the first of these (owing to some 

 suspicious readings) was carefully re-calibrated at the National Physical Laboratory 

 in December, 1910. 



These thermometers were of boro-silicate glass, and the depressions of the freezing- 

 point, after heating them to 100 C., ranged from 0'025 to 0'045, and some days 

 were required before the normal condition was again restored after heating. The 

 thermometers were not heated beyond 80 C., and generally not beyond 73 C. The 

 two lower-range thermometers were not heated beyond 55 C. The outflow ther- 

 mometer for the continuous-flow experiments was always used at a steady temperature 

 of about 55 C., so that no question of zero depression arose upon it. The inflow 

 thermometer was never heated above laboratory temperature. The outflow and 

 inflow thermometers, each of which only required to be read at one point, were 

 repeatedly compared with the standards under the conditions of actual use. All 

 thermometers were always used in the vertical position.* 



Electrical Standards. The electrical calibrations necessary for this investigation 

 were originally founded upon a cadmium cell (N.P.L. 5179) and a 1-ohm manganin 

 resistance (N.P.L. 5180). Special care was kindly given to the comparison of these 

 standards with those of the National Physical Laboratory. The international ohm 

 is based upon the resistance of a column of mercury 106 '3 cm. in length. The 

 international volt is such as to give the normal Weston cell, set up in accordance 

 N\ith the standard specification, a value of T0184 volts at 20 C. Dr. GLA/EBROOK 



* No tapping of the thermometers in use was required, as the apparatus had a stirrer always rotating at 

 a speed of 340 revolutions a minute, the vibrations of which were more effective than tapping. 



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