1002.] 



a Theory of the Capillary Electrometer. 



103 



ence has shown that capillaries are less liable to become sticky with 

 much acid than with very little. The only objection to this type is 

 the difficulty of adjusting the capillary in so small a trough. The 

 operation has to be effected under the microscope and is both delicate 

 and tedious. When it has been completed the definition is perfect, 

 and the instrument not unduly fragile. But in order to clean it the 

 same process has to be gone through again. 



I therefore determined to try an entirely different plan, on which 

 the safety of the capillary should depend not on the rigidity of the 

 supports by which the trough was fixed, but on their perfect flexibility 

 and on the use of a trough so light that its entire weight might even 

 be borne by the capillary. 



Fig. 1 is a perspective diagram of the 

 instrument in its final form, and figs. 2, 

 a, b, c, and d, show the details of the 

 trough which is the essential part. The 

 support A is cut from a solid block of 

 ebonite 9 cm. long, 5 cm. wide, and 

 2 cm. thick. It is first cut to shape, 

 holes drilled for the binding screws E 

 and F, and the piece B then separated 

 from it by two saw-cuts. 



V-shaped grooves are cut to receive 

 the capillary C, which is firmly clamped 

 under B by E and F. The longer limb 

 of the U-tube D passes through a hole 

 drilled lengthways through the lower 

 end of A, which is slit about half way 

 up with a wide saw-cut, so that it may 

 be pinched together by the screw G. 

 Adjustments for setting the capillary 

 at right angles to the optic axis and 

 parallel to the slit of the photographic- 

 recording apparatus, are provided for by 

 the stout brass plate K, bent at right 

 angles, one end of which is fastened by 

 a binding-screw at L to the back of A, 

 and the other by a similar screw M to 

 the adjustible stand of the projection microscope. The brass plate 

 K is so shaped that there is a space of about 3 mm. between it and 

 the left-hand side of the ebonite support A, in order to leave room for 

 the adjustment of the latter about L as a centre. 



The construction of the trough is shown in the full-size diagrams, 

 figs. 2, a, b, c. A piece of mica, such as is used for lamp-shades, is cut 

 to the shape a, with a pair of scissors. Two or three thicknesses may 



c 



Fig. 1.— Half full size. 



