654 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



circuit is closed from p to n by passing through the coil of the 

 bobbins E E. The excentric in moving to the right will open this 

 latter circuit, but will then close that which passes by E' E', 

 and then the soft iron of that electro-magnet will be magnetised 

 in its turn. 



So at each complete phase of the series the cylinders will be 

 attracted to the left and the right, and the reciprocating motion of 

 the rods and beam will be the result. 



The two movable cylinders always remain very close to the 

 interiors of the fixed cylinders ; this is rendered indispensable by the 

 known laws regulating the attractive force of magnets ; this force 

 increasing with extreme rapidity in proportion as the attracted and 

 attracting masses more nearly approach to contact. The beam also is 

 elongated by a good sized lever, so that the motion communicated to 

 the crank of the driving shaft may have a sufficient amplitude. 



II. ELECTRO-MOTORS WITH CONSTANT KOTATION. 



We now pass to the type of electro-magnetic engine which 

 gives directly a continuous motion of rotation. We may take 

 Froment's electro-motor for an example. Fig. 426 gives its general 

 aspect. 



Six pairs of electro-magnets, of which the figure only represents 

 four, in order that the movable wheels and their armatures may be 

 seen, are arranged along the radii of a circle, and are fixed to the 

 framework of the engine carrying the driving shaft, which has its 

 axis horizontal and passing through the centre of the same circle. 

 Wheels concentric with this carry eight armatures of soft iron, ar- 

 ranged parallel to the axis of rotation, and which in the course of 

 the motion place themselves in pairs with regard to the poles of 

 the electro-magnets. 



The eight armatures being distributed at equal intervals round 

 the circumference of the movable wheel, and the number of electro- 

 magnets similarly distributed being only six, when two armatures are 

 exactly opposite the two electro -magnets E E, Fig. 427, the other 

 armatures will be in front or behind according to the direction of the 

 motion. Suppose this to be in the direction of the arrows or from 



