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



contact-maker on its shaft for the purpose of enabling the 

 curves of current of electromotive force to be delineated. A 

 brief description of the motor is as follows : — 



It consists of two sets of field magnets M, M (see fig. 1), 

 having eight poles on each side which are secured to two 

 cast-iron disks. Between these field magnets revolves a small 

 armature A, the iron core of which is formed of a very thin 

 strip of transformer iron wound up into a ring, the armature 

 coils being wound on this ring. The armature coils are joined 

 up in series with one another, so as to give a series of 

 alternating magnetic poles round the ring when a current 

 flows through the armature circuit. The diameter of this 

 armature is about six inches. The field magnets have eight 

 poles, and the armature eight coils. The field magnetic coils 

 are bobbins about 2 inches long and 1^ inch in diameter, and 

 when joined up in series in the proper manner the field 

 magnets take a current of about eight amperes to give them 

 a proper amount of saturation. The armature is carried 

 upon a hard wooden boss fixed to a steel shaft ; and the steel 

 shaft is carried through small ball bearings made like bicycle 

 bearings. In order to prevent any side shake of the armature, 

 there are at opposite ends of the base cast-iron pillars with a 

 gun-metal screw through each, against which the rounded 

 end of the shaft bears ; the position of the shaft can thus be 

 adjusted with great nicety, and runs with great freedom from 

 friction. The ends of the armature circuit are brought to 

 two small insulated collars fixed on the shaft, against which 

 press two light brass brushes marked B, B kept against the 

 shaft by means of an expanding steel wire W. The armature 

 shaft carries on one side an ebonite disk with a steel slip let 

 into it. Two insulated springs S, S are carried on a rocking 

 arm H; this rocking arm can be traversed through half a 

 circumference, and is centred upon the gun-metal screw 

 which prevents side shake in the shaft, and a pointer and 

 graduated scale enables the exact position of the contact- 

 springs to be determined. One of the stop-screws keeping 

 the shaft from side shake is pierced with a longitudinal hole, 

 and through this hole passes a stiff steel wire ; this serves to 

 drive an aluminium disk 27 centims. in diameter and 4 milli- 

 metres thick. This disk is carried on a shaft which runs in 

 a cast-iron bearing, and the disk is therefore driven syn- 

 chronously by the motor. This aluminium disk has four slits 

 in it separated by angular intervals of 90° ; the slits are 0*5 

 centimetre wide and 4*5 centimetres long. If the field- 

 magnets of the motor are excited by a continuous current of 



