ELECTRO-MAGNETIC INSTRUMENTS. 



Fig. 421. x)e La Rive's Ring (Fig. 42 1 .) This 



consists of a glass cylindrical vessel, having 

 a cork float attached to its upper end ; 

 into this vessel is inserted a small voltaic 

 combination, consisting of a plate of zinc 

 surrounded by a plate of copper ; the zinc 

 plate being insulated from the copper. 

 A copper wire attached to both these 

 plates is made into the form of a ring, con- 

 sisting of several coils of the wire, which is 

 insulated by being wrapped around with 

 thread. Upon pouring dilute sulphuric 

 acid into the vessels, and placing the plates 

 in it, voltaic action commences, and.is manifested by placing 

 the apparatus afloat in water, when the coil will have a 

 tendency to take a position in the plane of the magnetic 

 meridian, and will exhibit all the effects of the attractive 

 and repulsive tendencies described, when a strong magnet 

 is brought near it on either side. If the pole be presented 

 to it on the side where attraction takes place, the ring will 

 move towards it till it arrives at the pole, and then proceed 

 onwards in the same course, the magnet being held in the 

 axis of the ring, till it reaches the middle of the magnet ; 

 but there it seems inclined to stop ; and then, after a few 

 oscillations, it settles as in a position of equilibrium ; for if 

 purposely displaced by bringing it forwards towards the 

 other pole, it returns with a force which shows that it is 

 repelled from that other pole. Let us now withdraw the 

 magnet, and turning it half round, so that its poles are in 



