154 
Sir Humphry Davy on a 
plates of about two hundred square feet. In describing 
this phenomenon, I shall not enter into very minute details, 
because the experiments, which led to the discovery of it, 
are very simple, and, though more distinct with a large ap- 
paratus, yet it may be observed by the use of a pair of 
plates containing from ten to fifteen square feet. 
Immediately after Mr. Faraday had published his ingeni- 
ous experiments on electro-magnetic rotation, I was induced 
to try the action of a magnet on mercury connected in the 
electrical circuit, hoping that, in this case, as there was no 
mechanical suspension of the conductor, the appearances 
would be exhibited in their most simple form ; and I found 
that when two wires were placed in a basin of mercury per- 
pendicular to the surface, and in the voltaic circuit of a battery 
with large plates ; and the pole of a powerful magnet held 
either above or below the wires, the mercury immediately 
began to revolve round the wire as an axis, according to the 
common circumstances of electro-magnetic rotation, and with 
a velocity exceedingly increased when the opposite poles of 
two magnets were used, one above, the other below. 
Masses of mercury of several inches in diameter were set 
in motion, and made to revolve in this manner, whenever the 
pole of the magnet was held near the perpendicular of the 
wire ; but when the pole was held above the mercury be- 
tween the two wires, the circular motion ceased ; and cur- 
rents took place in the mercury in opposite directions, one to 
the right, and the other to the left of the magnet. These 
circumstances, and various others which it would be tedious 
to detail, induced me to believe that the passage of the elec- 
tricity through the mercury produced motions independent of 
