liaith Remarks on Mr. E. M. Clarke. 363 



coils are fixed. The front end of the spindle is, for the pur- 

 pose of insulation, made of ivory or hard wood, and the lancet- 

 shaped blades F are mounted on a copper wire, which passes 

 through the centre of the spindle, and to which one end of each 

 coil of wire is soldered. E is the copper disc which always re- 

 mains in contact with the mercury in the cup below, and is by a 

 simple contrivance brought into contact with either of the other 

 ends of the two coils. This contrivance is shown in fig. 5, which 

 is an enlarged and side view of the front end of the spindle : at 

 G, in the socket of the copper wheel, is a notch, in which termi- 

 nate one end of each of the wires A B and C D ; one side of the 

 notch is represented in contact with A B, or the long wire for 

 giving the shock, but by twisting the socket partly round the 

 other side of the notch it will be brought in contact with C D, 

 or the short wires for showing the brilliant spark, and pro- 

 ducing the strongest heating effect. The points F in fig. 4, 

 are in the proper position to take the spark from the coils 

 C D, provided the socket of the disc is in contact with C D. 

 To obtain the spark from the coil A B, which is however far 

 less bright than the former, the notch must be brought into 

 contact with A B, and the points twisted round one quarter 

 of a revolution, or to that position that they will leave the 

 surface of the mercury at the instant when the coils from which 

 the spark is to be taken arrive at their greatest distance from 

 the poles of the magnet. 



For obtaining the shock, igniting wires, decomposing water, 

 &c., the points should be removed, and the two ends of the 

 wire forming the circuit should be connected, one with the 

 mercury in the cup, and the other with the termination of the 

 wire which passes through the insulating end of the spindle. 



The action of the machine will be more readily understood 

 by confining the attention to a single circuit; for this purpose 

 we must suppose two of the cylinders (those opposite each 

 other) with their coils of wire to be removed. Each of the 

 soft iron cylinders becomes, from the known laws of induc- 

 tion, a temporary magnet when it is opposite one of the poles 

 of the permanent magnet : as each cylinder passes succes- 

 sively both poles of the magnet, its poles are changed twice 

 during each revolution, and the cylinders cease to be mag- 

 netic when they are at equal distances between the two poles. 

 Electric currents are induced in the coils round each cylinder, 

 and on account of the alternate change of the poles, these 

 currents are alternately in opposite directions. The part of the 

 coil round one cylinder being, as above described, connected 

 with the copper disc, and that round the other cylinder con- 

 2 U2 



