6 



Mr. E. H. Griffiths. The Value of [Feb. 16, 



II. " The Value of the Mechanical Equivalent of Heat, deduced 

 from some Experiments performed with the view of 

 establishing the Relation between the Electrical and Me- 

 chanical Units, together with an Investigation into the 

 Capacity for Heat of Water at different Temperatures." 

 By E. H. Griffiths, M.A., Assistant Lecturer, Sidney 

 Sussex College, Cambridge. Communicated by R. T. 

 Glazebrook, F.R.S. Received January 19, 1893. 



(Abstract.) 



The paper of which this communication is an abstract gives the 

 particulars of an investigation which was commenced in the year 1887, 

 and extended to the close of 1892. The object of the enquiry is 

 sufficiently indicated on the title page. 



The values of the mechanical equivalent obtained by Joule in his 

 later determinations differ amongst themselves by as much as 1 per 

 cent., and the differences amongst tbe results obtained by succeeding 

 observers are, with the exception of Rowland's in 1880, still greater. 

 The harmony amongst the values obtained by Rowland is marvel- 

 lous ; but, since his manner of investigation was the same throughout 

 tbe whole series of his experiments, his conclusions stand in need of 

 confirmation by different methods of observation. Other observers 

 who have attempted to obtain the value of the mechanical equivalent, 

 by means of the work done by an electric current, have been hampered 

 by constant perplexities as to the absolute values of the electrical 

 units adopted. The science of electrical measurements has now 

 arrived at such a stage that its units may be regarded as sufficiently 

 established,* and, therefore, the time seems particularly appropriate 

 for an enquiry into the relation between those units and the 

 mechanical ones. 



The difficulties of such an investigation are, of course, great, as is 

 shown by the discrepancies amongst the results obtained by those 

 observers who, in recent years, have adopted electrical methods. One 

 cause of inaccuracy has been present in all determinations I have 

 examined, viz., the increase in temperature of the conductor above 

 the temperature of the medium in which it was placed, and the 

 consequent undetermined alteration in its resistance. Rowlandf 

 writes as follows : " There can be no doubt that experiments depend- 

 ing on the heating of a wire give too small a value of the equivalent, 

 seeing that the temperature of the wire during heating must always 



* < B. A. Report,' 1892. 



f 4 Proceedings American Academy,' June, 1879, p. 153. 



