404 A Portable Capillary Electrometer. 



solution. From this equation it follows : (1) that if is posi- 

 tive (mercury thread sloping upwards towards the meniscus) 

 the sensitiveness is not increased appreciably by making A 

 very large in comparison with a, unless is very small ; and 

 (2) that if 6 is negative (meniscus at lowest part of thread) 

 the mercury becomes unstable when 6 is numerically greater 

 than tan -1 (p 1 -h p 2 )a/(p ] ^p 2 )A } i. e. when 6 is numerically 

 greater than a/A approximately, since in order that the 

 polarization may be practically confined to the capillary 

 electrode, the ratio a/A must be small. It also follows 

 that if it were proposed to design an instrument which should 

 have the utmost sensitiveness possible, attention would have 

 to be paid to the straightness of the capillary as well as to 

 the uniformity of its cross-section. It is not necessary, 

 however, to take the precautions here indicated in order to 

 obtain the sensitiveness already quoted -, it may be obtained 

 with almost any capillary-tube chosen at random, and with 

 6 having such a positive value that the restoring force for a 

 small displacement is considerable enough to render the 

 instrument easy to work with. 



The sensitiveness of '0001 volt, which is obtainable with- 

 out any difficulty if the mercury is clean, is sufficient for a 

 great many measurements in which the electrometer can be 

 employed, and for these the electrometer (which for the pur- 

 pose in question is really a surface-tension galvanometer) is 

 more convenient than an ordinary galvanometer with a sus- 

 pended magnetic system. The electrometer is much more 

 easily set up than an ordinary galvanometer. There is no 

 suspension, no lamp and scale, and, practically, no levelling ; 

 but it is advisable when working with the instrument that 

 the potential of E x should never be more than a few tenths 

 of a volt less, or more than about a volt greater, than the 

 potential of E 2 . These conditions of working are, in general, 

 not difficult to satisfy. If the potential-differences applied 

 exceed either of the limits above mentioned, it is sometimes 

 necessary to run some of the mercury through the Capillary 

 tube in order to get the instrument again into its best working- 

 order. The capillary electrometer is already used almost 

 universally in physico-chemical laboratories, and the modi- 

 fications herein suggested may, perhaps, have the ertect of 

 slightly extending the sphere of its usefulness. The accom- 

 panying Plate X., taken from a photograph, represents one 

 form of the instrument described. Its size is about two-fifths of 

 that of the actual instrument. The arrangement of the different 

 parts will be obvious from the description already given. 

 The illumination of the end of the mercury-thread is effected 

 by means of a concave mirror attached to the base of the 

 instrument. 



