PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 209 



The agreement between the separate determinations by the mechauical 

 and by the electrical methods respectively, and the regularity of the 

 ■differences between Rowland's values and those of Griffiths, is such as to 

 raise a strong presumption that, in the experiments by both methods, 

 errors of observation have been reduced to a very small amount. At the 

 same time the difference between the two sets of results points to some 

 constant source of error in the measurement of energy affecting one or 

 both. The mechanical method is, in principle, so direct and simple that 

 it is difficult to suppose its results affected by a constant error. On the 

 other hand, the electrical method being less direct and more complicated, 

 there is here more room for uncertainty in the data. 



The electrical determinations depend upon the well-known relation 

 between thermal and electrical energy, which is expressible in the three 

 forms — 



Schuster and Gannon's experiments are based upon the second form 

 -of the equation, those of Griffiths on the third. In both of them electro- 

 motive force was measured by comparison with a Latimer Clark's cell. 

 Schuster and Gannon measured, in addition, the strength of their current 

 by means of a silver-voltameter, and Griffiths measured a resistance in 

 terms of the ohm. 



The accepted value of the electromotive force of the Clark's cell depends 

 in its turn on the electrochemical equivalent of silver as determined by 

 Lord Rayleigh and Professor F. Kohlrausch, and consequently it appears 

 that the electrical determinations of the mechanical equivalent involve a 

 double reference to the electrochemical equivalent of silver, so that any 

 inaccuracy in the adopted value of this quantity would involve a duplicate 

 ■error in the value of the mechanical equivalent deduced therefrom. 



In this connection it may be mentioned that, in a recent letter to 

 ^Nature,' vol. Ivi. p. 292, Lord Rayleigh has stated that he does not 

 ■consider that a possible error of one part in 1,000 is excluded from his 

 determination of the electrochemical equivalent of silver. If it be 

 assumed that his value is one part in 1,000 too small, this would almost 

 exactly account for the difference between the electrical determinations 

 into which this quantity enters as a factor and . the direct mechanical 

 'detei minations. 



It thus appears to be a matter of urgent importance that a redeter- 

 mination of the electrochemical equivalent of silver should be made, and 

 that the general question of the absolute measurement of electric currents 

 should be investigated. In order to enable them to carry out this investi- 

 gation, the Committee have decided to ask for reappointment and to apply 

 for a grant of 100^. towards the expense of the necessary apparatus and 

 •experiments. 



1897. 



