320 Prof. J. A. Fleming and Mr. J. E. Petavel : 



if sufficient speed be gathered, drop into step with the alter- 

 nating current driving it. Since the motor has eight mag- 

 netic poles, it makes one complete revolution in four complete 

 periodic times, so that if the motor is being driven from an 

 alternating-current circuit having a frequency of 100, then 

 the motor has to run at 1500 revolutions per minute before 

 it will drop into step, but at that speed it will fall into step 

 with the current passing through its armature, and will be 

 driven as a synchronous motor. Under these circumstances, 

 if a ray of light is passed transversely to the disk in such a 

 manner as to pass through the slits of the aluminium disk 

 during the progress of rotation of the disk, the beam of light 

 will be interrupted, but will obtain passage four times during 

 each revolution through the slits in the disk as it goes round. 

 If the motor is being driven by the same alternating 

 current circuit which supplies the alternating current to an arc 

 lamp, it is evident that, on looking through the slits in the 

 revolving disk at the alternating current arc, it will be seen in 

 one constant condition during its periodic variation, such 

 instant being determined by the position of the slits with 

 reference to the phase of the current. Without entering 

 into a longer description, it will be evident that this synchro- 

 nizing motor driving a disk and a contact-breaker enabled 

 two things to be done — first, to delineate all the current and 

 electromotive-force curves of the arc taken in the usual way; 

 and, secondly, a ray to be taken from the arc selected at one 

 particular instant during the complete period through which 

 the variation of illumination passes. These arrangements 

 were completed by the construction of a photometer of a 

 particular kind. Owing to the slow variation of position of 

 the electric discharge in the alternating current arc, it would 

 have been useless to photometer the instantaneous value of 

 the light coming from the alternating current arc against any 

 fixed standard of light ; but it was found possible to make a 

 very exact comparison between the intensity of the light 

 coming from any part of the arc, and selected at any one 

 constant instant during the complete phase, with the mean 

 value of the light coming from that same part of the arc 

 during the complete period ; in other words, it was found 

 possible to photometer the arc against itself, and so eliminate 

 to a large extent the difficulties arising from slow periodic 

 variations of the light sent out from the arc in any one 

 direction. It is well known that the light of an alternating 

 current arc, taken in any one direction, undergoes a slow 

 periodic variation quite independently of the variation of 



