of High Specific Resistances. 455 



made fast by screws to each plate, and brought as near to one 

 another as consistent with leaving an air insulating space 

 between them. These were then bored to fit slightly conical 

 steady pins. A rim was also screwed round the lower plate, 

 so that when the top plate was removed the bottom plate 

 resembled a tray. The rim projected about *8 of a centimetre 

 on each side of the surface, and extended to a height of about 

 half a centimetre above it. 



The gum was introduced between the plates in the follow- 

 ing way. The micrometer-screws were carefully cleaned and 

 screwed through their holes till they made contact with the 

 lower plate ; the point of contact was almost as easy to 

 estimate as in the ordinary use of the spherometer. At all 

 events three or four consecutive attempts to fix the point of 

 contact did not differ from each other by more than about one 

 half of one of the micrometer-divisions. The accuracy with 

 which the contact-point can be fixed depeuds mainly on the 

 workmanship of the screw, which must fit perfectly " tight 

 and free," to use the mechanic's very expressive term. The 

 contact-points having been found, they were permanently 

 scratched on the micrometer-heads and called zero points. 

 The next operation consisted in screwing each of the screws 

 through a known number of turns. In one experiment this 

 amounted to making the distance of the plates apart = '02 

 centim. ; and in another the distance was reduced to '01 

 centim. with equally good results. It is not advisable in any 

 absolute measure to reduce the distance to much less than 

 this, because the error of the micrometer, depending as it 

 does (as in this case) chiefly on the small irregularities of 

 the screw, must not be allowed to become sensible. There is 

 no doubt, however, that with first-rate appliances the micro- 

 meters might be easily made a hundredfold as accurate as mine, 

 and their travel actually .measured in situ by a suitable 

 reading-microscope. In this case it would be important to 

 make the screw portion much thicker to avoid any risk of 

 permanent distortion (twisting) when the screws are finally 

 screwed back while partly held by the gum*. 



The screws being adjusted, the plates are slowly heated in 

 a gas-oven till some gum laid on the surface of the lower one 

 is in a state of quiet fusion. The great object is to avoid 

 any distortion of the plates. With this aim in view the 

 plates described were cast about twice as thick as they finally 

 required to be in order that the " shell/'' supposed to be in a 



* Since this was written an improvement of the above micrometer- 

 screw has been devised, entirely getting rid of the difficulty here referred 

 to.— Oct. 3, 1889. 



