292 



LOCALIZED ELECTRIZATION. 



magnet turns. This axis carries a pinion, over which passes a 

 Yancanson's chain, carried by the large wheel E, which is itself 

 turned by a handle. 



"The electro-magnet is no longer, as in Pixii's instrument, 

 a single bar of soft iron curved into horse-shoe shape ; but 



consists of two distinct 

 reels, G and F, the soft- 

 iron centres of which are 

 united, on one side, by 

 a traverse of the same 

 material, and on the other 

 side by a traverse of a 

 material that is not mag- 

 netic. This arrangement 

 is equivalent to an electro- 

 magnet formed in a single 

 piece, but it is more easy 

 of construction, and less 

 cumbrous. 



"The analysis of the 

 currents produced by this 

 last apparatus is the same 

 as for that of Pixii's ; the 

 currents changing their 

 direction at each half- 

 revolution of the reels. 

 When it is desired to 

 have a current al ways in 

 the same direction, it is necessary to interpose, between the reel 

 and the place where the electricity is to be used, a contrivance 

 which has received the name of tlie Bedresser of the currents or 

 the Commutator. 



" The following is a description of this contrivance, in Clarke's 

 instrument : — A collar, H, formed of non-conducting material, is 

 mounted on one of the extremities of the axis on which the reels 

 turn ; and two metallic half-cylinders, separated from each other 

 by two small intervals at diametrically ojoposite points, are placed 

 upon the collar. Each of them is permanently connected with 

 one of the ends of the wire of the reels ; and two springs are con- 

 stantly applied over the half-cylinders. By the rotation of the 

 axis, each of the half-cylinders comes successively into contact 

 with each of the two springs. 



" When we wish to display physiological effects, we can employ 

 he current without the commutator; but the effects will then be 



Fig. 74. — Clarke's instrument. 



