174 MR, FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 
between bodies, relative to the intensity of the current produced or tending to 
be produced in them by magneto-electric induction, which might be shown by 
opposing them to each other ; especially as Messrs. Arago, Babbage, Hers- 
chell, and Harris have all found great differences, not only between the 
metals and other substances, but between the metals themselves, in their power 
of receiving motion from or giving it to a magnet in trials by revolution (130.). 
I therefore took two wires, each one hundred and twenty feet long, one of iron 
and the other of copper. These were connected with each other at their ends, 
and then extended in the direction of the magnetic meridian, so as to form 
two nearly parallel lines, nowhere in contact except at the extremities. The 
copper wire was then divided in the middle, and examined by a delicate galva- 
nometer, but no evidence of an electrical current was obtained. 
184. By favour of His Royal Highness the President of the Society, I ob- 
tained the permission of His Majesty to make experiments at the lake in the 
gardens of Kensington-palace, for the purpose of comparing, in a similar man- 
ner, water and metal. The basin of this lake is artificial; the water is supplied 
by the Chelsea Company ; no springs run into it, and it presented what I re- 
quired, namely, a uniform mass of still pure water, with banks ranging nearly 
from east to west, and from north to south. 
185. Two perfectly clean bright copper plates, each exposing four square 
feet of surface, were soldered to the extremities of a copper wire ; the plates 
were immersed in the water, north and south of each other, the wire which 
connected them being arranged upon the grass of the bank. The plates were 
about four hundred and eighty feet from each other, in a right line; the wire 
was probably six hundred feet long. This wire was then divided in the mid- 
dle, and connected by two cups of mercury with a delicate galvanometer. 
186. At first, indications of electric currents were obtained; but when these 
were tested by inverting the direction of contact, and in other ways, they were 
found to be due to other causes than the one sought for. A little difference in 
temperature ; a minute portion of the nitrate of mercury used to amalgamate 
the wires, entering into the water employed to reduce the two cups of mer- 
cury to the same temperature; was sufficient to produce currents of electricity, 
which affected the galvanometer, notwithstanding they had to pass nearly five 
hundred feet of water. When these and other interfering causes were guarded 
