ELECTRO-MAGNETIC INSTRUMENTS. 23 



and the pole of a magnet is placed in contact with the ex- 

 ternal end of the iron wire, the movable wire within rapidly 

 rotates round the temporary magnet thus formed by induc- 

 tion at the moment, and by changing either the connexion 

 or the pole of the magnet in contact with the iron, the di- 

 rection of the motion itself is changed. Price, $3.50. 



Revolving Magnet. F} - 442 - 



This instrument (Fig. 

 442) consists of a mag- 

 net, about seven inches 

 long, having at each 

 end a delicate point, 

 on which it is sup- 

 ported by a mahogany 

 stand, pillar, and arch- 

 ed frjtrae-work, and 

 turning at the bottom 

 in a small brass cup, in 

 which there is a well- 

 finished centre, and at 

 the top in a centre at- 

 tached to the lower 

 part of the screw-cap. 

 Near the middle of the 

 magnet there is sup- 

 ported, by an arm from the pillar, a stage in the form of a 

 ring, through the centre of which the magnet passes freely, 

 and carrying a cistern of mercury, which also surrounds the 

 magnet without touching it. A small wire, pointed and 

 amalgamated at its end, is fixed to the middle of the mag- 

 net immediately above the cistern, and is bent so as just to 

 dip into the mercury contained in the cistern ; a copper 

 wire, connected with the interior of the cistern, passing 

 through its side, and bent upwards, terminates in a cup, 

 holding mercury, for effecting the communication with the 

 voltaic battery by wires in the usual manner ; the other pole 

 being in contact with the cup at either the top or bottom of 

 the magnet, and in which there is also mercury, thus forming 

 a continuous circuit through either half of the magnet, as 

 the wires may be arranged, and rotating with considerable 

 velocity on its axis. Price, $5.00. 



