248 Mr. B. Hopkinson. [June 16, 



This method of measurement is only strictly accurate if K = A, in 

 which case the value of K should appear constant for all resistances. 

 For present purposes we may take the higher value 1'3 ohms (which is 

 the more nearly accurate) as a good enough approximation for (L + K )p, 

 or 1 -42 ohms if the additional reactance of the mains is included, and 

 0*5 ohm for up. 



The power required to drive the motor unloaded against friction, 

 hysteresis, etc. (taken as the applied watts less the C 2 R losses in the 

 armature), was found to average about 250 watts. This is the value 

 of JAjp/3 in all cases when the motor is described as unloaded. 



Finally the value of p was roughly determined in the following 

 way : The current was passed through the field coils, the machine 

 being at rest and the circuit of the armature open. The field coils 

 were short-circuited at a definite instant of time, and the current in 

 them was measured at various times after the short-circuit took place. 

 The short-circuiting was effected by means of a long heavy pendulum 

 which closed a switch at the lowest point of its swing. The pendulum 

 carried a contact maker which at a determinate later point in the swing 

 connected a resistance included in the field circuit to an electrometer. 

 The fall of potential in the resistance gave the current at the moment 

 of making contact, and the time could be calculated from the length of 

 the pendulum swing between closing the switch and making contact. 

 It was found that with sufficient accuracy for present purposes the 

 value of the current in the field magnets was halved in 0'07 second. 

 Since in this case the current diminishes according to the law e -'/>*, it 

 follows that /A is about one-tenth of a second ; /* varies somewhat with 

 the temperature of the field coils, etc., one-tenth can be taken as its 

 order of magnitude. 



Taking the above values of the constants, it appears that with the 

 motor connected direct to the mains y is negative. Furthermore, unless 

 the machine is very much under-excited, we have 



taking 60 v /(2) volts as a rough value for (A - A.a )jt?, p as 0'8 ohm, and 

 ^/[L (L 4- ^)].pas 1 ohm, so that y is also negative unless /3 has a value 

 comparable with 30 \/(2) amperes. We should, therefore, expect this 

 machine to be violently unstable. This was in fact, found to be the 

 case ; when connected direct to the mains as a motor with an exciting 

 current of thirty or more the machine very rapidly developed oscilla- 

 tions, and finally dropped out of step altogether. The oscillations 

 occurred just the same, whether a large or small generator was working 

 at the Power Station. They were not affected by putting heavy rings 

 of copper round the pole-pieces in addition to the field winding. It 

 was only by reducing the field current to 13 -5 and loading the machine 



