Electro- Magnetic Apparatus and Experiments. 259 



A, is a short piece of copper wire soldered to e, and descending 

 into the mercury cup n, which is soldered to the brass strap h. 

 The brass ball /, is movable on the projecting screw o, and serves 

 as a regulator to the vibrations of the electrotome. The circuit 

 traversed by the galvanic current is as follows. From the cup c, 

 by the dotted line to the brass strap b', thence through m, e, A, n, 

 b, to one of the large wire terminations. The other termination 

 of the large wires surrounding the magnet, is soldered to a cup 

 connected with c'. When the galvanic circuit is completed, the 

 magnet attracts the ball g, 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 g, 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 mercury7 "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 length of time. 



Circular Galvanometers. 



Figures 2 and 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 to a 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 concentric with the coil c. 

 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 c, 

 of insulated copper wire, is fastened by strong cement to the pil- 

 lar d. p, n, are the terminations of the coil passing into the mer- 

 cury cups on the stand. The coil is made of a number of strands 

 of wire in lieu of a continuous wire. Galvanometer coils are 

 usually 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 primitive current from an elementary battery, strengthens 



