IN VOLTAIC ELECTRICITY AND ELECTRO-MAGNETISM. 
295 
box along with it. Two small metallic points were made to pass from the 
lower copper ring through the bottom of the box, for the purpose of dipping 
into a. cup for holding mercury. The opposite sides of the upper ring were 
connected by a wire, to the middle of which was soldered a metallic point to 
dip into a small metallic cup for holding mercury on the top of the magnet. 
The whole arrangement will be obvious from the simple inspection of fig. 8, 
in which the cylinder C is seen in perspective, with the metallic point to dip 
into the mercury contained in the wooden cup which is used for the common 
rotation of a wire, and N the magnet. The box is partly suspended by an 
untwisted thread T, so that the metallic point may not rest on the bottom of 
the cup on the top of the magnet, but simply dip into the mercury. 
When the wire iv, from the cup on the top of the magnet, is connected with 
one end of a powerful battery, and the wire from the wooden cup into which 
the metallic point dips, connected with the other, the water in the box is rapidly 
decomposed, and the whole revolves about the pole of the magnet. By changing 
the poles, the box and its contents are made to turn round in the opposite 
direction. I was now anxious to make the hollow cylinder of water revolve, 
whilst the vessel in which it was contained remained stationary. This was 
accomplished by the following arrangement. 
Exp. XII. Two glass cylinders were cemented into grooves in a circular piece 
of wood, through the centre of which the magnet was made to pass, as in the 
preceding experiment. The circular rims of copper were fixed as in the wooden 
cylinder, the breadth of the upper ring being considerably less than that of the 
lower. The inspection of fig. 9. will render the whole obvious, in which A B 
is a section of the glass cylinders, N the magnet, W the wire connected with 
the lowest copper ring, W' that connected with the other and reaching to 
the ends of the battery ; V represents a wooden vane, having two vertical 
branches immersed in the conducting liquid, and balanced on a fine point 
resting on the top of the magnet. 
When the wires are connected with a powerful battery, the water begins to 
revolve, forming a real vortex and carrying the wooden vane along with it. 
When bodies of the same specific gravity as that of the fluid are thrown 
into it, the rotation is rendered obvious without the wooden vane. When the 
lower ring is connected with the negative end of the battery, the bubbles of 
2 q 2 
