RATINGS AND GUARANTEES. 163 



lead may be found by trial for which the condition of sparkless 

 commutation above stated, is completely realized. 



(b) The use of high resistance brushes of carbon tends not only 

 to cause a decrease to zero of the current existing in the sections 

 before they reach a brush, but also to establish the necessary 

 reversed current during the time that a section is short-circuited 

 by the brush. The decrease to zero is due mainly to the resis- 

 tance of the brush material which forms a part of the short 

 circuit. The establishment of the reversed current in the sections 

 is due mainly to the contact resistance between the brush and the 

 commutator, as follows : when the commutator has moved a little 

 beyond the position shown in Fig. no, the increased resistance 

 between bar e and the brush, due to decreased area of contact, 

 diverts the current a through the section V as a reversed current 

 in V. 



It is impracticable to depend solely upon the action described 

 above under (a] for producing sparkless commutation, inasmuch 

 as to do so would require the lead of the brushes to be read- 

 justed with every change of load. Therefore the action described 

 above under (b) must be depended upon to a greater or less 

 extent. 



In the case of generators and of motors which are driven 

 always in the same direction, the brushes are adjusted to give the 

 best commutation at, say, half load, and at greater (or smaller) 

 loads the action (a) is deficient (or excessive) and the deficiency 

 (or excess) must be counteracted by action (&). 



In motors for cranes and electric cars the direction of running 

 is repeatedly reversed and the brushes are fixed permanently in 

 the diagonal axes AB, Fig. 106. In this case the cross-magne- 

 tizing action of the armature current shifts the neutral axis so 

 that the action (a) tends to maintain rather than to reverse the 

 current in the short-circuited sections. In this case the action 

 described above under (V) is not only the sole cause of the re- 

 versal of the current in the short-circuited sections, but in produc- 

 ing this reversal the action (b) has to counteract the opposing 



