ELECTRO-MAGNETIC INSTRUMENTS. 29 



Fig. 449. 

 Fig. 448. 



fixed on its circumference, having a spindle supported by 

 two brass pillars and a brass plate to the poles of an electro- 

 magnet, wound as usual, and fixed to a mahogany circular 

 base ; at one end of the spindle there is a break- piece, 

 formed of a disc of metal, and having four silver pins pro- 

 jecting therefrom, their positions corresponding to those 

 of the armatures on the wheel ; against these pins there is 

 a silver spring, touching the pins in their revolution, which 

 is connected with one of the cups with binding screws at the 

 base, the wires of the electro-magnet being connected with 

 the other cup and the frame of the mngnet in such a manner, 

 that the electro-magnet will be charged on the approach of 

 any one of the armatures, but cut otf as it recedes there- 

 from, the current being formed by the contact of the projecting 

 pins in the break-piece against the spring, and broken by 

 their separation, the momentum the wheel having acquired 



