INDUSTRIAL APPLICATIONS. 



is greater than 50 per cent. This efficiency is nearer unity the 

 greater the velocity and the greater the coefficient of self-induction. 



1282. In order to rectify currents in the outer circuit a com- 

 mutator is used, formed, for instance, of two half-rings B and B', 

 connected respectively with the two ends of the induced wire, and 

 of two springs b and b' forming the termination of the external 

 circuit. The springs should pass from one half to the other of 

 the ring at the moment the current is null; with this object the 

 diameter of the contacts bb' must be displaced in the direction of 

 the motion through an angle 2ir<J>, starting from the position which 

 the diameter of the section occupies at the moment the electromotive 

 force is null. 



If these conditions were exactly realised, the mean intensity I' 

 qf the external current (535) would be 



No sparks should be produced in the commutator; but they 

 cannot be entirely avoided, for the current varies very rapidly in 

 passing through zero, and at the moment of the inversion each of 

 the springs is for an appreciable time in contact with the two half 

 rings. The machine is then short circuited ; there is accordingly 

 a loss of energy by the heating of the circuit, and a production of 

 sparks on breaking contact. The inconvenience of rectifying the 

 current is due less to the loss of energy than to the rapid waste of 

 the commutator. 



1283. Let us suppose that the external circuit contains an 

 electromotive force E' ; the differential equation becomes 



(6) 



When this electromotive force is constant, like that of a battery, 

 the intensity of the current for a permanent regime is expressed by 



E' 



. 



JX 



The induced current simply adds itself with its sign to the current 



