1902.] 



a Theory of the Capillary Electrometer. 



105 



tilted back during each alteration. When these adjustments have 

 been made, the inside of the trough is wetted by touching it with a 

 glass rod dipped in dilute sulphuric acid of 25 per cent., and the 

 apparatus is tilted forwards until the wet trough swings against the 

 capillary, and sticks to it. A piece of thin cover-glass — or of mica if 

 very high powers are to be used — slighly wider than the trough, is 

 picked up with a pair of fine forceps, wetted on one side with the acid, 

 and placed carefully against the trough, to which it adheres, holding 

 it firmly against the capillary, the lower edge of the glass resting 

 against the platinum loops with which the trough is fastened together 

 (see fig. 2, c). 



The U-tube is then turned back into position, cautiously raised until 

 the lower edge of the glass just dips into the acid, and clamped by the 

 screw G. Finally the trough is gently shaken or pushed to and fro in 

 the plane of the mica, until the acid rises in it to the required height 

 and all bubbles are expelled. 



The trough is held together so firmly by surface-tension that it seems 

 at first sight a difficult matter to take off the cover-glass without 

 breaking the capillary. It may, however, be done with the greatest 

 ease as follows : The screw G is loosened, and the U-tube D drawn 

 down and turned aside. A small beaker filled with water is held in 

 its place and raised until the trough H is completely immersed, when 

 the slightest movement causes the glass to fall off'. The apparatus is 

 then tilted back so that the trough swings clear of the capillary, which 

 may be washed or even wiped, and the trough dried, replaced, and a 

 new cover-glass put on, in less than three minutes. 



The definition, with these electrometers, is perfect. The capillary 

 touches the cover-glass throughout its length, so that any dry objective 

 can be used. The microscope should not, however, be left focussed 

 on the capillary, lest the acid should chance to get between the objec- 

 tive and the cover-glass — an accident which I have known to happen 

 in very damp weather. 



My only fear in designing this instrument was lest contact 

 with the mica should contaminate the acid and so spoil the tubes. 

 There does not, however, seem to be any such effect. I have had 

 some mica troughs in use for nearly three years, and have never once 

 been troubled with a sticky capillary, and even with induction shocks 

 they will stand more than the old type on account of the larger 

 quantity of acid in the U-tube. They are easier to make than my 

 old " normal type " on account of the straight capillary, and so far as 

 I can judge they seem likely to supplant both it and my previous 

 projection electrometer. 



