178 THE DIRECT-CURRENT MOTOR < II. VIII 



power. If a force factor of 3'5 kilodynes is required at 

 the start and M is constant, the motor must be capable of 

 carrying 450 amperes, and will weigh about 5,300 pounds. 



The force factor curves afford a convenient comparison 

 between dynamos of different types. Thus in Fig. 13, force 

 factor curves are given for the four-pole type already 

 referred to, and for a set of two-pole dynamos of 125 volts 

 induced tension. Since equal force factor and equal speed 

 give equal power, we see that a four-pole dynamo weighs 

 less than a two-pole dynamo of the same power, and that 

 for equal weight the four-pole machine carries more current 

 than the two-pole machine, but runs at a slower speed, 

 because the ratio of the diameter to the width of the 

 armature is greater in the four-pole than in the two- 

 pole type. 



The saving in weight is due to the fact that the 

 four-pole type carries more current per pound than the 

 two-pole type. It might appear that since the four-pole 

 type has to run at a slower speed for the same weight 

 than the two-pole type, it is at a disadvantage, and that if 

 it could run quicker there might be a still further saving 

 in weight. But this is not really so. 



A given power means a given current for given induced 

 tension. It is the current that determines the weight, and 

 it is here that the four-pole type has the advantage, 

 carrying more amperes per pound. Now the weight deter- 

 mines the speed for each type, and the speed determines 

 the induction factor. If a machine of any type and weight 

 could run quicker, it would simply mean that the induction 

 factor might be less, but this would not lighten the machine, 

 since the weight is determined by the current. The only 

 difference would be that tbe force factor would be less. 



