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BEXJ. PIKE S, JR., DESCRIPTIVE CATALOGUE. 



revolve with great rapidity as soon as the contacts are 

 made with the battery ; for this purpose, however, the wheel 

 must dip so far into the mercury as that each of the rays 

 shall touch the surface before the preceding ray has quitted 

 the mercury. The direction of the motion depends, of course, 

 on the same circumstances as were before mentioned. Mr. 

 Barlow observes, however, that in general the experiment 

 succeeds best when the wheel revolves inwards. 



Flg> 44 * This instrument is also 



constructed with two 

 wheels, as represented in 

 Fig. 440. An electro- 

 magnet is sometimes sub- 

 stituted for the steel mag- 

 net, being included in the 

 circuit with the wheel, 

 the current flowing through both. Changing the direction 

 of the current in this arrangement, changes the direction of 

 the rotation. 



Price, $2.50. 



" with two wheels, $4.00. 



" " " " and electro -magnet, $5.50. 



" one " " " " $3.50. 



Fig. 441. ^4 Wire revolving round a Magnet. (Fig. 



441.) This apparatus consists of a glass cylin- 

 der having at the lower end a cup of wood, 

 through which a small rod of iron is fastened, 

 projecting through for a short distance into the 

 glass tube, and surrounded with mercury. The 

 other end of the iron rod is inserted in a wood 

 base, having four short pillars, beneath which 

 a magnet may be placed in contact with the 

 iron rod, inducing magnetism therein. On the 

 top of the cylinder is a brass cup, having a screw 

 cup through which a piece of platinum wire 

 passes, and terminates below by a loop ; another 

 piece of wire hangs from this by a loop, and its lower end, 

 which dips a very little way into the mercury ; being amal- 

 gamated, it is preserved from adhering either to the iron wire 

 or to the glass. When even a feeble voltaic combination is 

 connected with the upper and lower ends of this apparatus, 



