20 Mr. G. Bird's Observations on induced Electric Currents 



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piece of hard wood, A and B, each having two holes excavated 

 in it for the purpose of holding mercury: each of these holes 

 communicates by means of thick copper wires, D D\ with that 

 opposed to it in the other piece of wood at the opposite end 

 of the board. Midway between these receptacles for mercury 

 is a wooden support, so contrived that a piece of soft iron wire 

 J inch in diameter and 5 inches in length may oscillate be- 

 tween the cheeks cut in its upper part, with as little friction 

 as possible, the iron wire being supported by milled-headed 

 screws. Around this iron wire E F, are wound two helices 

 of thin insulated copper wire, in the same direction, (from 

 right to left,) in such a manner that the two ends of one helix 

 may terminate in the copper points G H, and the ends of the 

 other helix in the points K L. Two small horse-shoe mag- 

 nets (not shown in the figure) are then fixed on proper sup- 

 ports, so that they may each be placed near an end of the 

 iron bar E F in a vertical plane just posterior to it, so that 

 on depressing the end F of the bar it may be opposite one 

 pole (say the south) of one magnet, and consequently the 

 end E will be opposite the other pole of the second magnet. 

 On elevating the end F, the contrary will of course take place, 

 and for this purpose it is hardly necessary to say that the 

 similar poles of the magnets should be in the same direction. 



From this description it is evident, that on connecting the 

 cups of mercury in A or B with the two plates of a single 

 voltaic battery, the bar E F will become a temporary magnet 

 if the ends of either helix are allowed to dip in the mercury ; 

 and if connection with the battery is properly made, the ends 

 or poles of the temporary magnet will be repelled by the poles 

 of the permanent magnet to which they are opposed ; the bar 

 will consequently move, and so cause the immersion of the ends 

 of the second helix in the other cups of mercury, repulsion will 

 again occur, and so on : about 300 oscillations of the iron bar 

 can be thus obtained in a minute. On connecting the ends 

 of the thick helix of the coil before described by tlie battery 



