38 
M. DE LA RIVE’S RESEARCHES ON THE VOLTAIC ARC. 
plate being positive and the point negative. As soon as the electro- magnet was 
charged, a sharp hissing was heard ; it became necessary to bring the point of the 
plate nearer to enable the arc to continue, and the bluish circular spot which the pla- 
tinum plate presented, became larger than when the experiment was made beyond 
the influence of the electro-magnet. The plate was made negative, and the point 
positive; the effect was then totally different ; the luminous arc no longer main- 
tained its vertical direction when the electro-magnet was charged, but took an 
oblique direction, as if it had been projected outwards tow r ards the margin of the 
plate ; it was broken incessantly, each time accompanied by a sharp and sudden 
noise, similar to the discharge of a Leyden jar. The direction in which the luminous 
arc is projected, depends upon the direction of the current producing it, as likewise 
on the position of the plate on one or other of the two poles, or between the poles 
of the electro-magnet. A plate and a point of silver, a plate and a point of copper, 
and generally a plate and a point of any other metal, provided it be not metal too 
easily fused, present the same phenomena. 
Copper, and still more silver, present a remarkable peculiarity. Plates of these 
two metals retain on their surfaces the impression of the action that took place in 
the experiments just described. Thus, when the plate is positive, that portion of its 
surface lying beneath the negative point presents a spot in the form of a helix ; as if 
the metal melted in this locality had undergone a gyratory motion around a centre, 
at the same time that it was uplifted in the shape of a cone towards the point. 
Moreover, the curve of the helix is fringed throughout by minute ramifications, 
precisely similar to the tufts which mark the passage of positive electricity in a 
Leyden jar. When the plate is negative and the point positive, the marks are totally 
different, being merely a simple point, or rather a circle of a very small diameter, 
whence proceeds a line more or less curved, forming a kind of tail to the comet, of 
which the small circle might be the nucleus : the direction of this tail depends upon 
the direction in which the luminous arc has been projected. 
When, instead of a plate and a point, two points are used for electrodes, it is evident 
that no visible trace of this phenomenon can be obtained ; but both the sharp hissing 
and the detonations may be produced, which latter are sometimes so loud as to bear a 
resemblance to distant discharges of musketry. For this the electro-magnet must be 
very powerful, and the current which produces the arc very intense. I had observed 
that when I took for a positive electrode a point of platinum, and for a negative elec- 
trode a point of copper, and placed them between the two poles of the electro-magnet, 
the production of the voltaic arc between the two poles was accompanied by a sharp 
hissing noise; whereas in the opposite case, the copper being positive, and the pla- 
tinum negative, the detonations were heard, attended by a frequent breaking of the 
arc. On examining this phenomenon more closely, I perceived that the fact I have 
just mentioned was due to the platinum becoming heated much more rapidly than 
the copper when they were employed as electrodes in producing the voltaic arc ; and 
