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Thisconstant, now designated as Joules equivalent, is the principal experimental datum of the science of thermodynamics.

measure is equivalent to 4.177 joules per calorie at 16.5° C., on the scale of Joule's mercury thermometer.

The unit of heat assumed in the table is the calorie at 20° C., which is taken as equal to 4.180 joules, as explained in the article Calorimetry.

Assoc. Report, 1899, with a slight modification Specific Heat Of Water In Terms Of Unit At 20° C. 4.180 Joules to allow for the increase in the specific heat below 20° C. This was estimated in 1899 as being equivalent to the addition of the constant quantity 0.020 to the values of the total heat h of the liquid as reckoned by the parabolic formula (5).

10° 15° 20° 2 5° 3 O ° 35° Joules Per Cal.