366 Prof. Rijke on the Duration of the Spark which 
Let AB be an isolated conductor of such length that a charge 
of electricity requires a perceptible time, say 7’, to traverse it. 
CA B 
Let C.D be another conductor much shorter than the first, and 
so placed that the extremities C and A of the two conductors are 
at the distance of several millims., the other extremity D of the 
second conductor being in communication with the ground, 
Now suppose that at a given moment a certain quantity of 
electricity be communicated to the extremity B of the conductor 
AB. At the end of 2” this electricity will have spread over the 
whole surface of the conductor. At the same instant, supposing 
the tension sufficient, a luminous discharge will commence be- 
tween A and C. We say will commence, admitting“therefore that 
the discharge will occupy a certain time. In fact, in order that 
the conductor might discharge itself instantaneously, it would be 
necessary that all the electric fluid should be accumulated at A. 
This, however, is not the case; since at the moment when the 
discharge commences, the whole surface of the conductor is 
occupied, though unequally, with electricity. Now it is easy to 
see that the electricity, which at the moment we are considering 
is still at B, will only reach the other extremity 7" afterwards* ; 
and even then it must be carefully noted that ad/ the electricity 
at B will not have passed to A; only part will have done so, and 
the rest will have remained at B, of which a further portion will 
reach A 2¢" after the discharge commences, and a third 3¢"' after 
the same epoch, and so on, 
The above reasoning shows that, during the time oceupied by 
the discharge, successive portions of electricity will arrive at A 
from B at equal intervals of time; and it is clear that the same 
will be the case with the electricity which at the beginning of 
the discharge was at any intermediate part of the conductor, 
only that the intervals of time will be shorter in proportion as 
the part considered is nearer A, It is therefore evident that 
there will be a continuous current of electricity towards this ex- 
tremity, and that the discharge will therefore be equally con- 
tinuous. ‘The passage of the electricity from A to C will of 
course cease immediately the tension at A descends below a cer- 
tain limit; but, on the other hand, it must not be forgotten 
* Tf, as some physicists believe, the rapidity of the propagation of elee- 
tricity diminishes with its density, then the electricity at B would require 
more than ¢" to arrive at A. 
