680 ALEXANDER MACFARLANE AND P. M. PLAYFAIR ON THE 









Measurement of the Difference of Potential required to pass a Spark through a 
Liquid Dielectric at the Atmospheric Pressure between Parallel Metal Plates 
at Different Distances. q 
TABLE I.-—Paraffin Oil, 14th June 1878. 


Length of Spark Difference of Mean rae 
in Centimetres. Deflection. Zero. Potential. 
S) N. n'. v—n. Potential 
(405 458 53 
: | 390 E 68 oo 
“ 315 456 141 
A 325 458 133 \ a 
§ 245 , 213 
: | 245 ‘ 213 — 
160 bs 298 
$ 180 : 278 
5 Escape 
It was impossible to obtain the reading for ‘5 centimetres; because the 
escape of electricity from the positive conductor caused the image to be driven 
off the scale before any discharge through the liquid took place. These 
observations when plotted (see diagram 1) lie very exactly on a straight line, 
which, however, has a small negative intercept on the axis of ordinates. From 
them we deduce— | 
V = 750 s—15, 
where V denotes relative difference of potential, and s the length of the spark. 
Hence— 4 
that is, the electrostatic force is constant. In absolute measure R = 364 C.G. 5 
units. f 
On repeating our experiments with oil of turpentine, we got observatiiag 
agreeing with the above and with our former results. 
