260 HENRY A. KOWLAND 



Surface density (a): 



a is obtained from electrometer-readings. 



V 



V *-f i/ 



A 



A = corrected area of movable plate 

 f=*r{5im 



.: V = 10 X 1'756 D iJ~uT, 



and ff = 1-397 - VaT. 



e ' 



As soon as the attempt was made to electrify the apparatus, diffi- 

 culties of insulation were met with. The charged system was quite 

 extensive, and the opportunity for leakage was abundant; in addition, 

 the winter here has been very damp. Most of the trouble of this kind 

 has been due to the glass in the apparatus; in no case where glass was 

 used as an insulator has it proved satisfactory, not even when the air 

 was dry. First, the stand with glass legs, on which the Ley den- jar 

 battery was placed, was found to furnish an excellent earth-connection. 



Paraffin blocks interposed stopped this. The reversing-key had 

 three glass rods in it, all of which were found to leak ; six different spec- 

 imens of glass, some bought particularly for this as insulating glass, 

 were all found to allow great leakage. Shellacing had no effect. Hard 

 rubber was finally substituted for glass ; and after that the key insulated 

 very well, even in damp weather. 



On charging the glass plates, the disks being earthed, it seemed 

 almost as if there was a direct earth-connection, so rapid was the fall of 

 the charge. This was not regarded at the time, as the plates were 

 always kept earthed ; but later, when it became necessary to charge the 

 plates, the insulation had to be made good. 



Investigation showed that this was caused by leakage directly through 

 the substance of the glass to the brass back-pieces (H H). Hard rubber 

 pieces were substituted, and the trouble was entirely removed. 



There was at first a deflexion in reversing the electrification while 

 the disks were at rest. This was of course due to direct electrostatic 

 effect; but it was not for some time clear where the point of weakness 

 in the electrostatic screen lay. It was found to be the faulty contact 

 between the tinfoil covering of the glass tube and the brass collar; the 

 brass had been lacquered. After this was corrected there was never 



