MM. Va-ii Beek, ^Van Rees and, Moll. 
other plate. The whole is suspended m equilibrium to some 
silken untwisted thread XZ. It may be observed, that the dis- 
tance P S, ought not to be so large as drawn in the figure, and 
care should be taken to prevent the spiral twisted round the 
qi]ill-tube from touching the wires P and S. The plates are now 
dipped in dilute acid, and the whole is suspended at X. Now, if 
a strong luagnet is brought ne^r T or X,. it will show a strong 
polarity, by its attraction or repulsion. Thus, the apparatus, 
with no other galvanic-battery but the ^ two small plates, shows 
the same phenomenon for which M. Ampere uses the instru- 
ment represented in Plate ii. Fig. 3. ^ of that gentleman’s paper 
on electro-magnetism, and which requires a pretty strong galvar- 
nic force. 
Another instrument delineated by M. Ampere, Fig. 2. Plate iii. 
may be more easily constructed thus : A brass-wire AC, Fig. 8/ 
rests at its bent end A in a cup containing some mercury, and 
is very moveable in azimuth round this point. The other end 
C, passes through the centre of a circular piece of thin paste- 
board, and then forms spiral turnings round this circular piece. 
The wire is attached by linen or any thread to the disk, the 
diameter of which, in my instrument, is about eight decimeters, 
there being about fourteen windings of the spiral. To its end 
B is suspended another wire, whose end reaches again tile sur- 
face of the mercury in the small cup D. The v/ires of the two 
poles of a galvanic-battery are in contact with the mercury in 
the cups A and D. It is required that the end of the wire A, 
on which the apparatus rests, should have a very free motion 
on a point placed in the middle. The plane of the disk will 
place itself in a situation perpendicular to the magnetic meri- 
dian, when the cups A and D communicate with the opposite 
poles of the battery. 
I received, some days since, the apparatus lately contrived 
in England by Mr Faraday, of which I have not yet found a 
description in any of the Scientific Journals, but which I expect 
to find in your next number. This little apparatus works to 
admiration. No other shows so clearly the magnetism of the 
comieGting wire. For if a strong magnet is brought near the 
* See this Journal^ Yol, IV., Plate VIII. Fig. 3. 
t See this Journal^ Vbl, IV,, Plate VlII. Fig, T. 
