412 Electro-Magnetic Experiments. 
pole of the needle to the west; by a current passing from north to 
south, the north pole of the needle will be deflected to the east. The 
galvanometer was covered by a glass receiver to prevent agitation 
from currents of air. The magnet used was the one alluded to above 
as supporting fifty pounds, this is a horse-shoe magnet, made of ten 
magnetic bars, the keeper being filed off in front applied itself to two 
of these. 
The magnet being placed with its poles to the south, (the north pole 
being westward) the keeper was brought to the magnet in such a po- 
sition that the coil was directed above the cylinder from north to 
south; below from south to north, the wire from the upper side was 
dipped into the mercury of the south cup of the galvanometer, the 
wire from the under side into the north cup. , 
On drawing the keeper from the magnet, the north pole of the 
needle of the galvanometer passed to the west; the needle being al- 
lowed to return to the meridian, a return of the keeper to the mag- 
net deflected the north pole to the east. By assisting the needle in 
its vibrations by the alternate withdrawal and return of the keeper, a 
very considerable vibration was produced. | 
By changing the keeper from right to left so that the wire passing 
over the cylinder was directed from south to north, the communica- 
tions with the galvanometer remaining as before, the reverse of the 
deflections just described took place. 
The effects described may be represented bya galvanic current 
circulating around the keeper, and at right angles to its axis, direct- 
ed from north to south above the cylinder when the keeper was with- 
drawn, in the contrary direction when it was returned. Such a cur- 
rent would evidently have passed out of the upper wire on the with- 
drawal of the keeper, and out of the un- 
der wire on its return; that is, would 
have passed from south to north, above 
the needle of the galvanometer, in the 
first case, and from north to south in the 
second. 
Fig. 2 shows the apparatus for pro- 
ducing vibration in the needle of the 
galvanometer by the wire coil and mag- 
net. A, B,is a roll of wrapped wire 
wound about a wooden spool, which 
has the central part removed ; the ends 
of the wire are connected with the wires 
