3 I2 



ELEMENTS OF ELECTRICAL ENGINEERING. 



through two of the resistances r in series, as shown by the short 

 arrows, whereas the useful current which enters or leaves the 

 armature through the brush must 

 flow through two of the resist- 

 ances r in parallel. When the 

 brush touches but one commu- 

 tator bar there is no short circuit, 

 and the whole of the useful cur- 

 rent must flow through one 'of 

 the resistances r. The resist- 

 ances r are entirely idle except 

 at the moments when the bars to which they are connected pass 

 under a brush. 



Resistance leads are objectionable because they involve a loss 

 of power, and because of the difficulty of finding room for them 

 in a compact armature structure. While a motor is in actual 

 operation the resistance leads come into service in rapid suc- 

 cession, and the heat is distributed among all of the leads ; at 

 starting, however, there is a possibility of the motor being stuck 

 fast, so that a given resistance lead may be in circuit for an indef- 

 c inite time. In view of this possibility 



0000 ^ * S necessarv to ma ke the resistance 



leads of large current-carrying ca- 

 pacity, and to place the leads so that 

 they can be easily repaired in case of 

 their being burned out. The resistance 

 leads are usually made of heavy strips 

 of German silver thoroughly insulated 

 and placed in the bottoms of the arma- 

 ture slots. 



Balanced choke coils. The essen- 

 Fig ' 269 ' tial features of the balanced choke 



coil arrangement for preventing sparking may be seen in Fig. 

 269, in which c is a section of the armature winding, ss are 

 two commutator bars and tt' are two choke coils through which 





