576 Mr Barlow’s Electro-Magnetic Experiments. 
time will acquire a velocity exceeding very considerably any of 
the motions hitherto described. 
“ It is unnecessary to say, that, by changing the contact, or 
by inverting the magnets, the direction of the rotation will be 
also changed. The usual precaution of covering the surface of 
the mercury with weak dilute nitric acid, will increase the rapi- 
dity of rotation, but it is not actually necessary in this case.” 
Exp. VIII. “ To exhibit the terrestrial directive quality of a 
galvanic wire. 
u M. de la Rive’s apparatus for this purpose consists of a small 
galvanic combination attached to a cork ; the plate of zinc is 
nearly half an inch wide, and extends about one and a half or 
two inches below its cork, its upper end passing through the 
same ; the slip of copper is of equal width with the zinc, but passes 
round it, being thus opposed to both its surfaces, as in Dr Wol- 
laston’s construction ; its upper end also appears through the 
cork. A piece of copper-wire, covered with silk thread, is coiled 
five or six times, and tied together so as to form a ring about 
an inch in diameter, and the ends of the wire are connected, by 
solder, one with the zinc, and the other with the copper slip 
above the cork. See Fig. 10. 
“ When this small apparatuses placed in water, slightly aci- 
dulated with sulphuric or nitric acid, the ring becomes highly 
magnetic, and will arrange itself in a plane perpendicular to the 
magnetic meridian, or it will at least indicate a tendency to take 
that position, but the escape of the bubbles, arising from the 
decomposition of the water, prevents it from preserving a fixed 
‘direction. 
“ Its magnetic qualities, however, are more obviously shown 
by bringing to it a strong magnet. The one I made use of is 
cylindrical, about three quarters in diameter, and 18 inches in 
length. This being applied at the distance of several inches, 
the ring was immediately attracted, or repelled, according as 
one or other of the poles of the magnet was presented, or 
according as one or other side of the wire was opposed to the 
latter. When the result of the application is attraction, the 
cork will advance towards the extremity of the magnet, and if 
