Instruments and Experiments. 



Ill 



together, does not affect the result. To show this fact, he used 

 a magnet loaded at its lower extremity with platinum, and float- 

 ing in a vertical position in a vessel fnll of mercury. The instru- 

 ment represented by figure 3, illus- 

 trates the same fact without the in- 

 convenience of using a large quan- 

 tity of mercury, and, in conse- 

 quence of the diminished resist- 

 ance to be overcome, exhibits a 

 much more rapid rotation than can 

 be obtained by means of Faraday's 

 apparatus. A magnet, pointed at 

 both ends, is supported on an agate 

 cup A, while its upper end is kept 

 in place by slightly entering a small 

 cavity in the lower extremity E of 

 a small brass rod passing up through 

 the arched top of the sustaining 

 brass frame-work, and surmounted 

 with a cup for making connection 

 with a voltaic battery. From one 

 side of the same rod, a copper wire 

 passes down into a small cistern 

 for containing mercury, resting on 

 the shoulder of the magnet near 

 its upper end. Two copper wires 

 projecting into this cistern, descend 

 into another of ivory, supported on 

 a stage, and surrounding the mid- 

 dle of the magnet, but not touch- 

 ing it. One end of a large bent 

 copper wire projects into the inte- 

 rior of the ivory cistern, and the 

 other supports a cup for making 

 communication with a battery. On putting a proper quantity 

 of mercury into the cistern, and transmitting a voltaic current 

 through the wires, the whole movable part of the apparatus will 

 rotate with considerable velocity. 



Boston, November 7, 1840. 



