The subject is necessarily technical because it involves not only the mechanical factors speed and torque but also the electrical quantities voltage, current and ?ux. Moreover, any or all of these five fundamental quantities may, in fact usually do, vary and are affected by other-factors or conditions. Hence the problems must be analyzed and solved with thoroughness to obtain results of real value and cannot be properly treated in a popular manner. Nevertheless care has been taken to introduce and explain each step or result as clearly as possible, and to illustrate each case, when feasible, by a specific numerical example based upon standard commercial motors.