610 



Mr. J. Viriamu Jones 



[May 24, 



axle by well-paraffined ebonite (Fig. 4), otherwise there would be 

 short-circuiting of part of its radius through the bearings and bed. 



It was at first intended to drive the disc by rope gearing from the 

 electromotor ; but in the course of preliminary experiments, though 

 the rope was 40 yards long, and joining of the ends took place over 

 a length of at least 6 feet, there was a sudden variation in speed, 

 producing a distinct movement of my galvanometer needle, and 

 visible at the tuning fork, of which I shall presently speak, whenever 

 the joint passed over the pulley. The rope gearing was therefore 

 abandoned, and the motor coupled direct. The current driving the 



a Ebonite. 

 v . 3) Q Paraffin Wax, 



Fig. 4. — Details of Disc Insulation and Brushes. Part Elevation and part 

 Section on lines a b, b c, Q Q 1 = brushes. 



motor is supplied from secondary cells. It passes to the motor 

 through resistance coils, and may be increased or diminished by 

 throwing some of these coils into or out of the circuit. It may also 

 be varied continuously through a small range by a slide resistance of 

 platinoid wire after the larger adjustment has been made. A shunt 

 worked by a lever provides means of taking out or putting in a small 

 resistance suddenly, so as to allow the observer controlling the speed 

 while an observation is being taken to counteract small variations of 

 speed due to alteration in the lubrication of the bearings and the 

 friction of the brushes on the commutator of the motor. With these 



