491 



Dr. H. H. Pcole on the Electrical 



LogK 



slate bench, thus protecting the end of it carrying the 

 galvanometers from leakage currents from the high-pressure 

 source. Without these precautions currents as large as 

 10 ~ 2 microampere were indicated by G l when no conducting 

 specimen was present. With them the current never 

 exceeded 2xl0~ 4 , which was negligible except in the cases 

 of paraffin -wax and shellac. 



Mica, 



Tests were made on a specimen (A) which had already 



been tested by the previous method (Phil. Mag. July 1916). 



This consists of a sheet 0*0173 mm. thick, with tinfoil disks 



1*15 cm. in diameter gummed to its opposite faces. An iron 



Fiar. 3. 







Z/ 



'JO 



/ 



/ O 















/ 









Mica Specimen A 



guard-ring gummed to one face prevents surface leakage 

 from causing errors. The free surface of the mica is coated 

 with shellac. 



