236 OUR PHYSICAL WORLD 



and attached to narrow copper strips that lie on opposite sides 

 of the axle in the plane of the unit. This pair of copper strips is 

 completely insulated from the next adjacent pair. This series 

 of metal plates borne on the axle is known as the commutator. 

 The current enters and leaves the units by two copper strips 

 that are applied on opposite sides of the commutator. 



Instead of using permanent magnets the commercial motor 

 uses electromagnets. A part of the current entering the motor 

 is shunted off through a branch wire, which is wound about a 

 horseshoe-shaped core, transforming it into a magnet. The 

 cylinder of rectangular units, known as the armature, revolves 

 on its axis between the poles of this magnet. The explanation of 

 the rotation is the same as that given for the simple unit; but no 

 sooner has one unit revolved so that its strips on the commutator 

 have lost contact with the strips that supply the current than 

 another unit receives the current. This unit is forced to rotate 

 in the same direction, and so the armature continues to revolve. 



There are many types of commercial motors, which differ 

 from each other principally either in the method of winding the 

 wire on the armature, in the arrangement of the coils, or in the 

 arrangements of the magnets. Each of these various types 

 possesses certain advantages, and each is adapted to a particular 

 sort of work. Some of them will run only on an alternating 

 current, others on a direct current. This point will be better 

 understood after the discussion of magnetos and dynamos. 



One other simple type of motor may be described that is often 

 used in schools as a demonstration motor, and that is sold in 

 shops as a toy. Imagine a rimless wheel, with three equidistant 

 iron spokes, to be so mounted that it will whirl between the ends 

 of a horseshoe-shaped electromagnet (Fig. 101). The current 

 flowing into the motor is divided. Part of it goes through a 

 wire that is wound first about one arm of the horseshoe-shaped 

 iron in an anticlockwise direction, and then in a reverse manner 

 on the other arm of the iron. Thus the current makes one pole 

 the north pole and the other the south pole of the electromagnet. 



