436 On the Mechanical Equivalent of Heat and the Ohm. 



error duo to this cause has in these later experiments been 

 determined and allowed for. Five experiments were made, 

 and the corresponding valnes of " v " calculated from the 

 results. These values are as follows: — 



299-9 x 10 8 

 300-3 x 10 8 

 299-4 xlO 8 

 298-0 xlO 8 

 299-9 xlO 8 



Mean value . . . 299-5 x 10 8 . 



LI. Note on the Relation between the Mechanical Equivalent of 

 Heat and the Ohm. By L. B. Fletcher, Student in Phy- 

 sics, Johns Hopkins University*. 



A SINGULAR error occurs in a paper, published in the 

 Philosophical Magazine (April and May 1880), by Dr. 

 C. R. A. Wright. After remarking that Joule's value of the 

 mechanical equivalent of heat, derived from experiments on 

 the heat generated by a measured current in a wire of known 

 resistance, is probably, when corrected for the error in the 

 resistance-estimation due to superheating of the wire, from 

 1*5 to 2 per cent, higher than Joule's water-friction value, 

 Dr. Wright goes on to say (p. 264) that : — " This difference 

 between the two values is precisely that which would subsist 

 did an error to an equal amount exist in the B.A. resistance- 

 unit valuation: i. e. if the B.A. unit were 1*015 to 1*020 

 earth-quadrant per second instead of being exactly 1 earth- 

 quadrant per second, the value of J deduced from Joule's 1867 

 experiments would be 1*015 to 1*020 times the true value; 

 for it is calculated by the formula 



T C 2 R* 



H 



where C is the current, R the resistance, t the time, and H 

 the heat evolved." This statement is evidently incorrect; for 

 if the ohm is really 1*02 earth-quadrant per second, and was 

 assumed by Joule to be exactly 1*00 earth-quadrant per 

 second, Joule's value for the resistance of his wire, and conse- 

 quently his value for J obtained by this method, must be 2 

 per cent, too small. 



* Communicated by the Author. 



