DOMESTIC MOTORS. 



493 



pie device termed a commutator. This has various forms in different 

 machines, but the principle involved is the same in all. The ends of 

 the wire of the revolving coil are connected with the halves of a cylin- 

 der separated by an insulating substance. A metallic brush composed 

 of bundles of wire, or thin strips, presses against each of these sections, 

 and, so long as the cylinder remains stationary, the current taken off 

 by the brushes will be an alternating one ; but, when the cylinder re- 

 volves with the coil, the brushes will change from one half to the other 

 at the moment of the reversal of the current, and its direction in the 

 circuit will, therefore, always be the same. In most machines the 

 armature has many coils, and the commutator cylinder a correspond- 

 ing number of insulated sections. 



The interest in the electric light during the past few years has 

 resulted in greatly improving such machines, and in the devising of 

 many new forms of varied excellence. A descrij)tion of one of these, 

 that has attracted wide attention and proved one of the most efficient, 

 will suffice to indicate the general construction and mode of action of 

 such devices. When a magnet is inserted in a closed coil of insulated 

 wire, a momentary current is induced in the coil, and, when it is with- 

 drawn, one of opposite direction occurs. If, instead of withdrawing 

 the magnet, it is passed through the coil, currents will be induced in 

 each of its spirals as the magnet passes them, which will be in one 

 direction during the passage of the first half of the magnet, and in a 

 reverse one during that of the latter half. If two magnets be placed 

 end to end with their like poles in contact, and be bent into the form 

 of a ring, currents can be continuously produced by revolving the ring 

 within an inclosing coil. Mechanical difficulties prevent such an ar- 

 rangement ; but, if, instead of permanent magnets, a ring of soft iron 

 be wound with insulated wire, and 

 revolved between the poles of a 

 magnet, the same results will be 

 obtained and the difficulties avoid- 

 ed. The iron ring becomes a mag- 

 net by induction, and as it revolves 

 its poles shift in the reverse direc- 

 tion, so as to always remain oppo- 

 site those of the inducing magnet. 

 The effect is the same as if the ring 

 remained stationary and the coil 

 revolved. This is the arrangement 

 of the armature adopted in the 

 Gramme machine, and the differ- 

 ence between it and others lies in this mode of inducing the current, 

 instead of by insertion and withdrawal of a magnet from a coil. The 

 manner in which the armature is actually constructed is shown in 

 Fig, 15. The ring is composed of a bundle of soft-iron wires, about 



Fig. 15. 



