390 



MESSRS. V. H. VELEY AND J. J. MANLET ON 



SUPPLEMENTARY NOTE ON THE APPLICATION OF CAREY FOSTER'S METHOD FOR 



COMPARISON OF RESISTANCES. 



It was thought advisable to obtain certain measurements by CAREY FOSTER'S 

 method for the comparison of resistances, not only to serve as a check upon the rest 

 of the work, but also to ascertain if this method could readily be applied in the 

 case of a decomposable electrolyte. 



For this purpose the auxiliary coils were wound upon glass tube frames and soaked 

 in paraffin wax ; the frame was then enclosed in a box which was placed in a larger 

 one, the space of two inches between them being packed with cotton wool. Two 

 thermometers were introduced through the lids, so that their bulbs were in the 

 centre. The maximum change registered during an hour was never more than one- 

 tenth of a degree. 



The process of working was as follows : The ordinary method was first employed 

 to obtain the value for K X 10", then the auxiliary coils introduced into circuit and 

 the determinations repeated. It will be seen from the following table that the results 

 obtained by the two methods are concordant when due allowance is made for the 

 slight variation of temperature. 



TABLE VII. 



As a matter of experience it was found that for CAREY FOSTER'S method the 

 electrodes should be continually recoated with the platinum deposit, otherwise it was 

 difficult to ascertain exactly the point of minimum sound in the telephone. 



EXPERIMENTAL RESULTS. 



The approximate temperatures selected for observation were 0, 15, and 30 for 

 acids of percentage concentration from 1'3 to 50, but 0, 10, and 20 above this 

 strength, as the more concentrated acids are not only extremely volatile, but also 

 liable to decomposition. 



In Table VIII. the values are given for the specific resistances in true ohms, corrected 



