ON OUR PRESENT KNOWLEDGE OF THE 

 NUMERICAL VALUE OF THE MECHAN- 

 ICAL EQUIVALENT OF HEAT. 



1""*HE subject of the mechanical equivalent of heat may 

 be regarded as a meeting-ground for many of 

 the explorers in the world of science. It is capable of 

 approach by many different paths and by distinct methods, 

 and thus affords to investigators a means of testing the 

 accuracy of the observations by which their steps have 

 been guided. 1 The value of this central position increases 

 as our knowledge concerning it becomes more definite, and 

 my object in this paper is to briefly discuss the value of 

 the information we now possess. 



Although it would be difficult to overrate the import- 

 ance of an accurate determination of the numerical value 

 of the mechanical equivalent, it would be easy to under- 

 estimate the difficulties of the achievement. Any inaccuracy 

 in our measurements of temperature, in our values for^, or 

 in our conclusions as to the changes in the temperature 

 coefficient of the specific heat of water, tell with fatal 

 effect in such an investigation. 



Again, if the method of inquiry adopted is based on the 

 work done by an electric current when passing between 

 points at different potentials, we are in addition confronted 

 with the possibilities of small experimental errors (whose 

 cumulative effect may be serious) in the values of our 

 electrical units. 



It is impossible in the present article to do more than 

 glance at the work of a few observers. Those who wish 

 for a complete summary will find a historical table in the 



1 In the discussion which followed the reading of a communication 

 on this subject to the Royal Society in February, 1893, Lord Kelvin 

 called attention to the fact that the discovery of the error in the B.A. 

 ohm was due to the circumstance that the adoption of that unit as a 

 standard of resistance caused the investigator to arrive at a value of J 

 which was evidently outside the possible limits of that constant. 



