CH. VIII THE FORCE FACTOK 175 



to be 810 amperes, while that required for friction can 

 be shown to be 850 amperes, making a total current 

 at the start of 1,660 amperes and a force factor of 239 

 kilodynes. 



The weight of a dynamo, if the induction factor is 

 constant, is mainly determined by the current that it 

 has to carry, and may be taken as increasing very nearly 

 in direct proportion to the current. 



On the other hand, the speed of a dynamo is limited 

 by its weight, since for any given type of machine, the 

 weight determines the diameter of the armature, and 

 therefore the speed for the maximum permissible peri- 

 pheral velocity, so that if a number of machines of the 

 same type have to be made giving the same induced 

 tension, the heavier machines must have the higher 

 induction factors in order to give the same induced volts 

 at lower speeds. 



If the power of a dynamo is given, and the induced 

 tension, either as motor or generator, the current is then 

 fixed, being the quotient of the power by the tension. 

 The current being fixed, the minimum weight is also 

 fixed, for any given type of machine. Having obtained 

 the minimum weight, we can then find the induction 

 factor, as we know the highest permissible speed for a 

 machine of this weight. The induction factor is ob- 

 tained by dividing the given tension by the maximum 

 speed. We thus get the lightest dynamo that will give 

 the required power at the given tension. 



For equal currents, the weight of a dynamo in- 

 creases with the induction factor, but not in direct pro- 

 portion ; since for any given type the ratio of the induction 

 factor to the weight increases slightly with the weight. 



