Unipolar Induction. 181 



-seemed of sufficient interest to justify the attempt to obtain 

 both positive and negative results with an apparatus like that 

 described above, and to prove experimentally that the inner 

 cylinder really would charge up in spite of its being enclosed 

 by a metallic screen rotating with it. 



Another apparatus has accordingly been constructed in 

 which either the condenser or the solenoid could be set in 

 rotation. The present paper contains an account of the work 

 and a discussion of the theoretical significance of the result. 



§ 2. Apparatus. 



The condenser and solenoid are shown to scale in fig. 2, 

 which is largely self-explanatory. All metal parts, including 

 the base, were of brass. Guard-rings in line with the inner 

 cylinder were introduced in order to simplify the calculation. 

 Rotation was effected by means of a belt connected to a 

 motor placed about 2 feet away. 



FiR-. 2. 



C, brush ; D, slip-ring ; E, solenoid, 22*5 cm. long, 1150 turns of wire ; 

 F, condenser : outer cylinder 20-5 cm. long, 6'64 cm. in diam. ; inner 



cylinder 15-1 cm. long, mean diam. 1*54 cm., on amber rinj 

 Diam. of shaft, 1-27 cm. 



•s a 



The principal apparatus was earthed by a brush of brass 

 wires sliding on the knob B, and was otherwise insulated 

 (except for a resistance of 10,000 ohms accidentally left in 

 place between frame and earth) ; the outer cylinder of the 

 condenser is thus practically earthed at a point on the axis. 

 In the final arrangement the needle H communicating with 

 the inner cylinder consisted of a copper wire about 1 mm. in 

 diameter, and was connected to the wire I leading to the 

 electrometer by means of an insulated brush of brass wires 

 bearing partly on one side and partly on the other. This 



