458 Mr. Morris Owen on 



falling from the ceiling to the floor, a distance of 292 centi- 

 metres. The mass of the falling weight was 1035 grammes, 

 and /'was 6*45 cms. per sec. 2 



The nibbed specimens consisted of small disks of ebonite or 

 glass mounted on pieces of sulphur attached firmly to ebonite 

 rods about 5 centimetres long. The ebonite rods were 

 square in section and fitted tightly into a socket in an ebonite 

 plate attached to the lower end of a bent lever. The lever 

 was mounted near the wheel so that the specimen could be 

 made to press horizontally against the rim of the latter, the 

 pressure being produced by placing a known weight in a 

 definite position near the end of the horizontal arm of the 

 lever. 



The rub was effected by bringing the specimen evenly into 

 contact with the rim of the wheel at the moment when the 

 falling weight reached the floor. Just before the wheel 

 came to rest the specimen was separated from it so that, 

 neglecting the small quantity of f rictional work at the axle 

 of the wheel, the work done against the friction of the 

 specimen was equal to the kinetic energy of the wheel as 

 calculated above. 



(2) Measurement of the Charge on the specimen. 



Immediately after a rub, the ebonite rod supporting the 

 specimen was quickly removed from the socket in the lever 

 and suspended (by a long silk thread always attached to it 

 and passing over a pulley) at a height of about a metre above 

 a tall metal jar into which the specimen could be lowered. 

 The jar was placed inside and insulated by sulphur supports 

 from a larger earth-connected jar which served to shield it 

 from outside influences. The inner jar was connected, by a 

 thin wire passing through a sulphur plug in the wall of the 

 outer jar, to the upper plate of a parallel plate condenser 

 (whose lower plate was earthed) and to a Dolezalek electro- 

 meter. The wires leading to the latter passed through 

 cylindrical metal tubes connected to the earth, being supported 

 in the tubes by sulphur plugs at their ends. One terminal 

 of the electrometer was earthed, and by means of a mercury 

 cup key in a block of paraffin wax the other terminal could 

 also be earthed for the purpose of obtaining the zero on 

 the scale. The parallel plate condenser was used for 

 measuring the capacity of the system, but was generally left 

 connected through a mercury key to the jar during the 

 measurements. 



The electrometer-needle was suspended by a fine platinum 

 wire of about '01 mm. diameter. It was charged by a small 



