Sir Humphry Davy on a 
1 56 
became plane ; and rotation slowly began round the wire. 
As the magnet approached, the rotation became more rapid, 
and when it was about half an inch above the mercury, a 
great depression of it was observed above the wire, and a 
vortex, which reached almost to the surface of the wire. 
In the first experiments which I made, the conical eleva- 
tions or fountains of mercury were about the tenth or twelfth 
of an inch high, and the vortices apparently as low ; but in 
the experiments made at the London Institution, the mercury 
being much higher above the wire, the elevations and depres- 
sions were much more considerable, amounting to the fifth 
or sixth of an inch. Of course, the rotation took place with 
either pole of a magnet or either wire, or both together, ac- 
cording to the well known circumstances which determine 
these effects. 
To ascertain whether the communication of heat diminish- 
ing the specific gravity of the mercury, had any share in 
these phenomena, I placed a delicate thermometer above one 
of the wires in the mercury, but there was no immediate ele- 
vation of temperature ; the heat of the mercury gradually 
increased, as did that of the wires ; but this increase was 
similar in every part of the circuit. I proved the same thing 
more distinctly, by making the whole apparatus a thermometer 
terminating in a fine tube filled with mercury. At the first 
instant that the mercury became electro-magnetic, there was 
no increase of its volume. 
This phenomenon cannot be attributed to common electri- 
cal repulsion ; for in the electro-magnetic circuit, similar 
electrified conductors do not repel, but attract each other ; 
and it is in the case in which conductors in opposite states are 
