198 ELECTROMETRY. 



Dr. Warren de la Rue and H. Miiller* resumed these experi- 

 ments, replacing the electrical machine by a battery of high potential, 

 the number of couples in which gave directly the difference of 

 potential. The couples are formed of silver, silver chloride, sal- 

 ammoniac, and zinc; their electromotive force is about 1*03 volt. 

 The following table gives the results obtained between two discs : 



Difference of Potential g, . 



Force 

 V 



163 



155 

 152 



146 



142 



136 



130 



124 



120 



112 



The differences of potential are here a little greater than in the 

 analogous experiments of Sir W. Thomson. 



According to the same authors, the striking distance between a 

 point and a disc increases almost as the square of the difference of 

 potential, for distances between a fraction of a millimetre and one 

 centimetre ; for the distance of one centimetre it is 9,200 volts. 



The results obtained with air are so closely concordant that, with 

 a certain degree of approximation, differences of potential might be 

 measured by the striking distance at any rate, if these distances do 

 not exceed two centimetres. Beyond that the difference of potential 

 increases very slowly, if even it does not tend towards a limit a little 

 higher than 400 electrostatic units, or 120,000 volts. A serious error 

 would be made if we wished to estimate the difference of potential 

 of a machine by the length of its spark ; in like manner, we cannot 

 conclude from the length of lightning discharges, which is frequently 



* WARREN DE LA RUE and H. MULLER. Phil. Trans. Roy. Soc. t Vol. 

 CLXIX., pp. 55, 155; Vol. CLXXI., p. 65; Vol. CLXXIV., p. 447. 



