68 



Profs. Ayrton and Perry on the 



C: 



np 



z — nt 



_ np 

 a ' 

 But as this is negative it is necessary, in order that the shunt- 

 motor may govern, that the terminals of the motor should be 

 reversed. This will reverse the current round the field-magnet; 

 but, by hypothesis, not the residual magnetism, otherwise p 

 would also become negative. The core of the electromagnet 

 must, therefore, be of steel, so strongly magnetized that the 

 shunt-current does not reverse the polarity, and the permanent 

 magnet and rotating armature together form a brake-dynamo 

 while the shunt-coil and the armature will form the motor. 

 But this is simply one of the solutions (a theoretical one of 

 course and not a practical one) given by us on page 310 of 

 our original paper and illustrated in figure 10 on that page. 



The fact, however, still remains that in the case of the 

 supply condition being constant current, equation (11) can 

 be satisfied without the employment of a series demagnetizing- 

 coil; and all that we have to do to in order to make the solution 

 a practical one, that is to enable an ordinary field-magnet with 

 soft iron core to be employed, is to alter the equation so that 

 when the speed equation (12) is satisfied (11) leads to a 

 positive value of the current. This result may be obtained 

 by adding to the right-hand side of equation (11) a positive 

 constant term e numerically larger than np, for in that case, if 



z-\-a 



0= 



,__ e— np 

 a 

 for all values of A. Now such a positive term e will be 



introduced on the right-hand side of equation e, if some 

 accumulators of E.M.F. equal to e be introduced into the 



