On the Duration of the Spark of an Electric Discharge. 865 
found by Faraday, 0°73, and agrees with a determination made 
by Andréeff*, who obtained the number 0°6364. 
Jolly also determined the coefficient of absorption of liquid 
ammonia, and found it to be for 1 degree 0°00155, which is 
about half as much as that of air. 
LV. On the Duration of the Spark which accompanies the dis- 
charge of an Electrical Conductor. By P.. RisKe, Professor 
of Natural Philosophy at the University of Leydent. 
1. ‘ N J HEN a Leyden jar is discharged in the ordinary way, 
the spark produced may be considered as instanta- 
neous; its duration, at least, is so short that it has hitherto 
been found impossible to measure it even approximately. This, 
however, is no longer the case when the charge has to traverse a 
body which offers any considerable resistance, as, for instance, a 
copper wire half a mile long. In fact, Mr. Wheatstone found f 
that the spark obtained thr ough a copper wire of ;/;th of an inch 
in diameter, and of the length: above mentioned, lasted for about 
se oooh of a second. 
2. This result, if Mr. Wheatstone had published it by itself, 
would probably have been explained simply - the ground that 
electricity required precisely this time, viz. —1 th of a second, 
to traverse the length of wire in question. This explanation 
would, however, have been, to say the least, incomplete, since in 
the same series of a Mr. Wheatstone proved that elec- 
tricity really requires 1 th of a second to travel that di- 
stance ; and in order to reconcile these results, he considered it 
necessary to have recourse to a new hypothesis, and to suppose 
“that the diameter of the wire was not sufficiently great to allow 
the charge to pass through it except in a successive manner.’ 
3. In reflecting on this question, it appeared to me that the 
results obtained by the above-named illustrious physicist might 
be easily explained on known grounds; and that it was by no 
means necessary to have recourse to a hypothesis, in support of 
which it would be difficult to cite a single direct observation. 
We shall see that it is, in fact, easy to prove a priori the fol- 
lowing proposition :— 
The time required by electricity to traverse a given conductor 1s 
much less than that required to discharge that conductor. 
* Liebig’s Annalen, vol. ex. p. 1. 
+ Communicated by the Author. 
t “On some Experiments to Measure the Velocity of Electricity and the 
duration of Electrical Light,” Phil. Trans. 1834. 
