Determining the Mechanical Equivalent of Heat. 167 



average potential-difference within about J- of one per cent., 

 the instruments having recently been tested against our 

 standards at the Central Technical College. 2000 cub. centim. 

 of water were used in all the experiments, and were measured 

 with a graduated glass jar, fresh water being used in each 

 experiment. The weight of the glass vessel up to the water- 

 level was determined as 184 grammes, and the specific beat 

 being about 0*2, the water-equivalent is 37 grammes. The 

 weight of the manganin strip is 114 grammes, and the specific 

 heat being about 0*09, the water-equivalent is 10; the equiva- 

 lent mass of water used being therefore 2047 grammes. 



No. of 

 expt. 



Time in 

 seconds. 



Temperature. 



Amperes. 



Mean 



TOltS. 



Calories per 

 watt- second. 









r 





Initial. 



Final. 



Rise. 





8-634 





1... 



120 



18-40 



22-02 



3-62 



30 



0-2383 



2... 



180 



13-25 



18-70 



5-45 



30 



8-634 



0-2390 



3... 



180 



13-60 



19-00 5-40 



30 



8-648 



0-2367 



4... 



120 



12-97 



1653 3-56 



30 



8656 



0-2375 



5... 



120 



12-64 



16-26 



3-62 



30 



8-698 



0-2365 



6... 



120 



12-89 



16-49 



360 



30 



8-662 



0-2364 



7... 



120 



1211 



15-72 



3-61 



30 



8-666 



0-2368 



8... 



120 



12-10 



15-74 



364 



30 



8-642 



0-2395 



9... 



120 



13-13 



16-75 



3-62 



" 30 



8-692 



0-2367 



Mean 0-2375 

 Average deviation from the mean = -001=0-42 per cent. 



With this apparatus, then, we get in about 10 minutes, 

 including the experiment and subsequent calculation, a result 

 for the heat-equivalent of the watt-second ; and these results 

 have an average deviation from the mean, if several experi- 

 ments are made, of less than ^ of one per cent. This result, 

 as we have seen above, should not differ from the true value 

 by as much as one per cent. It may here be pointed out 

 that as the water taken from the ordinary water-supply is 

 sure to be at least two or three degrees below the atmospheric 

 temperature at the time, it is exceedingly easy to arrange 

 that its mean temperature during the experiment shall be 

 exactly equal to the temperature of the air, for no artificial 

 method of cooling is required, as would be the case if a 

 greater range of temperature were used. This, however, was 

 not done except in one or two of the above experiments, and 

 is not necessary to ensure the accuracy that was required. 



The mean temperature of the water in the above experi- 

 ments was 15 degrees Centigrade; and our final result is that 

 at that temperature 0*2375 gramme of water are raised 1° C, 

 in temperature by the energy of one watt-second. 



