338 



ELEMENTARY LESSONS ON [CHAP. vn. 



S T> 



structed electro-magnetic engines producing rotation by 

 electromagnetic means. Fig. 141 shows a modification 

 of Ritchie's electromotor. An electro -magnet DC, is 

 poised upon a vertical axis between, the poles of a fixed 

 magnet (or electromagnet) SN. 

 A current, generated by a suit- 

 able battery, is carried by wires 

 which terminate in two mer- 

 cury-cups, A, B, into which dip 

 the ends of the coil of the mov- 

 able electromagnet CD. When 

 a current traverses the coil of 

 CD it turns so as to set itself 

 in the line between the poles 

 ^o NS, but as it swings round, 

 ^&J \ _ the wires that dip into the mer- 



cury-cups pass from one cup to 

 the opposite, so that, at the 

 moment when C approaches S, 

 the current in CD is reversed, 

 ;.'".". and C is repelled from S and 



attracted round to N, the cur- 

 rent through CD being thus reversed every half turn. In 

 larger electromotors, the mercury -cup arrangement is 

 replaced by a commutator, consisting of a brass ring, slit 

 into two or more parts, and touched at opposite points 

 by a pair of metallic springs or " contact-brushes." 



In another form of electromotor, devised by Froment, 

 bars of iron fixed upon the circumference of a rotating 

 cylinder are attracted up towards an electromagnet, in 

 which the current is automatically broken at the instant 

 when each bar has come close up to its poles. In a third 

 kind, an electromagnet is made to attract a piece of soft 

 iron alternately up and down, with a motion like the 

 piston of a steam-engine, which is converted by a crank 

 into a rotatory motion. In these cases the difficulty 

 occurs that, as the attraction of an electromagnet falls off 



