Electro-Magnetic Apparatus and Experiments. 259 
h, is a short piece of copper wire epldensd to e, and distending 
into the mercury cup n, which is soldered to the brass strap 6. 
The brass ball J, is movable on the projecting screw 0, and serves 
as a regulator to the vibrations of the electrotome. The circuit 
traversed by the galvanic current is as follows. From the peg e, 
by the dotted line to the brass strap ’, thence through m, e, h, n, 
b, to one of the large wire terminations. The other termination 
of: the large wires surrounding the magnet, is soldeted to a cup: 
connected with & When the galvanic circuit is ‘completed, the 
magnet attracts the ball ¢, and raises e from m, producing a bright 
spark at m, and a powerful shock from d, d ; e, then falls by its own 
weight, re-establishes the connexion, and thus the vibration con- 
tinues. On the side of the ball 2, towards the pole of the mag- 
net, is fastened a piece of brass, or other non-magnetic substance, 
to prevent the adhesion of the ball to the magnet. The tips of 
the wires h, m, should ‘be tinned before use. In all cases, tin- 
ning, or covering - with soft solder the extremities of wires for 
connexions, and dipping them into mercury, will be found ‘a 
much more preferable mode of amalgamating, than the usual 
practice of dipping them into nitrate of mercury, as they pre- 
serve their brightness. a greater aa of time. 
- Circular Galvariometers. 
Figures 2and 3, represent two new forms of galvanometers, 
which are found to possess some advantages over other forms in 
common use. The whole appearance of this instrument, (though 
a trivial consideration ,) is somewhat in its favor for purposes of 
general exhibition toa class. a, fig. 2, is the magnetic needle © 
suspended by its centre on a fine point. The needle is made of - 
Watch spring, and bent into a form coneentrie with the coil ¢. 
7 
The distance between the poles of the needle is about one six- 
teenth of an inch more than the width of the coil. The coil ¢, 
of insulated copper wire, is fastened by strong cement to the pil- 
lard. p,m, are the terminations of the coil passing into the mer- 
“eury cups on the stand. _'The coil is made of a number of strands 
of wire in lieu of a continuous wire. Galvanometer coils are 
ustially made of too fine wire, and of a single wire of too great 
~a length. M. Pouillet, in his late investigation of the general 
law of the intensity of currents, has shown that derivation made 
upon a. _— current from an elementary battery, strengthens 
