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Lord Kelvin and Mr. Maorius Maclean on 



and one for the charging wire D. Both the water-dropper 

 and the charging wire, ending with a pin-point as sharp as 

 possible, are insulated by solid paraffin, which is surrounded 

 by a metal tube, as shown in half size in fig. 2. To start 



Fig. 2. 



TUBE OF- WATER- DROPPER 

 PARAFFIN 



METAL TUBE 



with they were supported by pieces of vulcanite embedded in 

 paraffin. But it was found that after the lapse of some days 

 (possibly on account of ozone generated by the incessant 

 brush-discharges), the insulation had utterly failed in both of 

 them. The vulcanite pieces were then taken out, and solid 

 paraffin, with the metal guard -tube round it to screen it from 

 electrically influencing the water-dropper, was substituted. 

 This has proved quite satisfactory : the water-dropper, with 

 the flow of water stopped, holds a positive or a negative 

 charge for hours. 



§ 6. A quadrant-electrometer E (described in ' Electro- 

 statics and Magnetism/ §§ 346-353) was set up on the top 

 of the vat near the water-dropper, as shown in fig. 1. Jt was 

 used with lamp and semi-transparent scale to indicate the 



