MR. GROVE ON THE ELECTRO-CHEMICAL POLARITY OF GASES. 
89 
made many other experiments on the voltaic arc taken in various gaseous media, 
with the view of ascertaining the state of the intervening media anterior to, during, 
and after the discharge ; these experiments have hitherto given me no results of any 
value. In the voltaic arc, the intense heat developed so affects the terminals and so 
masks the proper electrical eftect, that the difficulty of isolating the latter is extreme ; 
and I have latterly sought for some modified form of electric discharge which should 
be intermediate between the voltaic arc and the ordinary Franklinic discharge, or 
that from the prime conductor of a frictional machine ; for something, in short, which 
should yield greater quantitative effects than the electrical machine, but not dissipate 
the terminals, as is done by the voltaic arc. 
An apparatus, to which M. Despretz was kind enough to call my attention re- 
cently at Paris, seemed to promise me some aid in this respect. It was constructed 
by M. Ruhmkorff, on the ordinary plan for producing an induced current, viz. a coil 
of stout wire round a soft iron core, with a secondary coil of fine wire exterior to it, 
having an ingenious self-working contact breaker attached ; from the attention paid 
to insulation in the construction of this apparatus, very exalted effects of induction 
could be procured. Thus in air rarefied by the air-pump, an aurora or discharge of 
5 or 6 inches long could be obtained from the secondary coil, and in air of ordinary 
density a spark of one-eighth of an inch long. 
I procured one of these apparatus from M. Ruhmkorff ; the size of the coil portion 
of the apparatus is 6’5 inches long, 4 inches diameter; the length of the wires 
forming the coils are (I give M. Ruhmkorff’s measurements) stout wire, 30 metres 
long, 2 millimetres diameter, 200 convolutions ; fine wire, 2500 metres long, metre 
diameter, 10,000 convolutions. These measurements will only be taken as ap- 
proximative, and indeed the exact size is immaterial to the consideration of the 
experiments which I am about to detail. I will not give my experiments in the 
order in which I made them, as I should have to describe many fruitless ones, but I 
will place first that which I consider the most important and fundamental. 
1st. On the plate of a good air-pump was placed a silvered copper plate, such as is 
ordinarily used for Daguerreotypes, the polished silver surface being uppermost. 
A receiver, with a rod passing through a collar of leathers, was used, and to the 
lower extremity of this rod was affixed a steel needle, which could thus be brought 
to any required distance from the silver surface; a vessel containing potassa fusa 
was suspended in the receiver, and a bladder of hydrogen gas was attached to a stop- 
cock, another orifice enabling me to pass atmospheric air into the receiver in such 
quantities as might be required*. A vacuum being made, hydrogen gas and air were 
allowed to enter the receiver in very small quantities, so as to form an attenuated 
atmosphere of the mixed gas : there vvas no barometer attached to my air-pump, but 
from separate experiments I found the most efficient extent of rarefaction for my 
purpose was that indicated by a barometric height of from half to three-quarters of 
* See a figure and description of the apparatus at the end of this paper. 
MDCCCLII. 
N 
