36 



BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



Fig. 458. 



four cups, two on each piece, the distances of which from 

 each other may by means of these holes be varied at pleasure. 

 Short wires, a, a, a, a, proceed horizontally from the bottoms 

 of the cups, and serve as pivots round which the two wires, 

 w, w, bent twice at right angles, are made to turn at the 

 upper part of their vertical branches, having small holes 

 drilled through them for that purpose. These wires, thus 

 hung freely upon their pivots, carry on their upper ends 

 small weights, which, bringing the centres of gravity as 

 nearly as possible in coincidence with the points of suspen- 

 sion, enable them to be moved by a very slight force. Con- 

 ducting wires, proceeding from a voltaic battery, are then 

 inserted into the cups previously filled with mercury, in 

 such a manner that the galvanic current shall pass in the 

 same direction through both the parallel wires ; the moment 

 this is done, the wires move towards each other, even from 

 a distance of several inches, exhibiting a powerful mutual 

 attraction. When the currents are transmitted in directions 

 opposite to each other in the two wires, which they may be 

 made to do by transposing the communicating wires inserted 

 into the cups leading to one of the movable wires, while the 

 others are left as before, the movable wires immediately 

 recede from each other, manifesting a repulsion as powerful 

 as the attraction was in the former case. Price, $5.50. 



The Scintillating Circle. (Fig. 458, as above.) This 

 instrument consists of a steel rim cut on the face after the 

 manner of a file, and connected by metallic contact with 

 one of the cups ha\ r ing binding screws on the base ; the rim 

 is fastened to an immovable disc, in the centre of which is a 

 brass axis, carrying a small pulley, to which rapid rotation 



