168 Prof. Ayrton and Mr. Haycraft on an Apparatus for 



Now we know with considerable accuracy the equivalent 

 in ergs of one watt- second, which certainly does not differ 

 from 10 7 by more than one tenth of one per cent.; that is to 

 say, the international volt and ampere do not differ trom 10 8 

 and 10 -1 C.G.S. units of electromotive force and current by 

 more than that amount (see section on the recent history of 

 the electrical units). Hence the result we have obtained, the 

 error in which is less than one per cent., gives us at once a 

 value for the mechanical equivalent of heat with the same 

 accuracy. 



Since we find the equivalent of the watt-second in gramme- 

 degrees at 15° C. to be 0*2375, we have at once, taking 10 7 

 ergs equal to one watt-second, the mechanical equivalent of 

 heat in ergs per gramme-degree at 15° C. equals 4-211 x 10 7 . 



Reducing this to foot-lbs. at Greenwich per lb.-degree 0. 

 at 15° O.j it becomes 1408, or in foot-lbs. per lb.-degree F. at 

 59° F. it is 782. 



A series of four experiments made subsequently by some 

 students at the Central Technical College, in which the cool- 

 ing error was eliminated by making the mean temperature of 

 the water equal to that of the air, gave as their mean 0*2384 

 calorie per watt-second, or 4*195 x 10 7 ergs per gramme- 

 degree at about IS° C. Reducing this value to foot-lbs. at 

 Greenwich per lb.-degrees C. and F. respectively, we get 

 1403 and 779 foot-lbs. 



IV. Previous Determinations of the Mechanical Equivalent 



of Heat. 



It is of interest to compare these figures with some of the 

 more recent results obtained for the mechanical equivalent. 



Rowland's value for the mechanical equivalent in ergs per 

 gramme-degree at 15° C. is 4*189 x 10', which reduced to 

 foot-lbs. at Greenwich per lb.-degree C. is 1401, and in foot- 

 lbs, per lb.-degree F. at 59° F. is 778*3. Rowland used the 

 method of direct friction of water, and was the first to dis- 

 cover that the specific heat of water was a minimum at 30° C. 

 and varied one per cent, between 5° and 30° C. (Proc. American 

 Acad. 1879-80). 



Dieterici (1889) gives as the result of a determination by 

 the electrical method in which Bunsen's ice-calorimeter was 

 used, the number 4*244 X 10 7 . Correcting from the " legal '' 

 ohm employed by him to the international unit, we get the 

 numbers 4*232 X 10 7 , 1415, and 786 {Annalen der Physik, 

 vol. xxxiii. p. 417). 



Miculescu/s careful determination by the direct method in 



