70 
MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
Table III. 
Volts. 
Striking distance. 
inch. 
250 
o-ooioo 
500 
0-00225 
750 
0-00350 
1000 ' 
0-00482 
1500 
0-00820 
2000 
0-01233 
2500 
0-01700 
3000 
0 02200 
3500 
0-02700 
4000 
0-03225 
4500 
0-03775 
5000 
0-04325 
5500* 
0-04900 
6000 
0-05460 
6500 
0-06070 
7000 
0-06650 
7500 
0-07250 
8000 
0-07852 
Differences. 
Between 
volts. 
125 
0 
125 
and 
132 
1000 
338 
and 
413 
2000 
467 
and 
500 
3000 
500 
and 
525 
4000 
550 
and 
550 
5000 
575 
and 
560 
6000 
610 
and 
580 
7000 
600 
and 
602 
8000 
Additional length of spark for 
1000 additional volts. 
inch. 
y 0-00482 
J 
0-00751 
0-00967 
0-01025 
0-01100 
0-01135 
0-01190 
0-01200 
It appears that after a difference of potential 2000 volts has been reached, each 
additional increment of 1000 gives about the same additional length to the spark, 
but not exactly so, as each succeeding number is slightly in excess of its predecessor 
up to 8000 volts at all events ; so that the striking distance for 8000 is 16 "29 times 
that for 1000. 
In December, 1859, and in the first four months of 1860, experiments were made 
under the direction of Sir William Thomson with the object of determining “the 
electro-motive force required to produce a spark.” An account of these experiments 
was presented to the Royal Society by Sir William Thomson, and published in the 
Proceedings.! It appears from this account that a condenser was used which might be 
* This agrees with Sir William Thomson’s conclusion (Proc. Roy. Soc., vol. x. p. 338), “that a 
Daniell’s battery of 5510 elements can produce a spark between two slightly convex metallic surfaces at 
2 * 0 th of an inch asunder in ordinary atmospheric air.” 
t Proc. Roy. Soc., vol. x. pp. 326-338, 1860. 
