Capillary and Electrical Phenomena, 285 



The Capillary Electrometer. 



Since, in the apparatus just described, every compensating 

 pressure is represented by a definite value of the electromotive 

 force interposed between a and (3 } the latter can be inferred from 

 the former when once a table is constructed for this purpose. 

 Thus the free electrical tension at the poles of a Daniell was 

 measured by the compensating pressure of 260 millims. mercury, 

 and smaller electromotive forces by smaller pressures. The ap- 

 paratus in this case is not traversed by a current ; for the electro- 

 motive force of polarization holds in equilibrium the electromo- 

 tive force between a and ft. It is only necessary to have this 

 electromotive force so small that it produces no appreciable de- 

 composition of water ; it must not much exceed one Daniell. This 

 can always be attained if a known number of DanielPs elements 

 be allowed to act against the electromotive force to be tested. 

 The capillary electrometer is also very sensitive in regard to 

 quantities of electricity; to charge it is to produce an ap- 

 preciable polarization upon a meniscus the surface of which is 

 equal to (2irx T J- D -) 2 = 0*0006 sq. millim. And in fact a deflec- 

 tion is obtained when a feebly charged proof plate from a Cou- 

 lomb's balance is approached to the pole « — or if the current of a 

 Daniell be momentarily closed by the apparatus, the current being 

 enfeebled by the interposition of a piece of air-dried glass. The 

 sensitiveness of such an electrometer is therefore so great that 

 that of no other electrometer, excepting that of Thomson, can 

 be compared with it. This sensitiveness is moreover constant 

 at all times ; the deflections are apparently momentary, in most 

 cases at least ; and the deadening is very rapid. The pole ft 

 must be in metallic connexion with the gas- or water-pipes, 

 while a is carefully insulated from the table by means of 

 shellac supports. If also a remains insulated from ft for some 

 time, the apparatus charges itself, as is also observed in Thom- 

 son's electrometer. Accidental contacts, atmospheric electricity, 

 &c. produce these charges. 



When the indications of an atmospheric-pressure manometer 

 are sufficient for measuring the pressure, the electrometer may be 

 arranged in a form which is easily transportable. The drawn- 

 out glass tube is only a few centimetres long, and is connected 

 with a second tube sealed at one end, which contains mercury and 

 compressed air, and acts as a manometer for atmospheric pres- 

 sure. This connexion is established by means of a short thick 

 caoutchouc tube, which is under a small screw-press to produce 

 the necessary pressure. The whole is placed upon a small 

 wooden bracket, which may be clamped to the stage of any mi- 

 croscope. The indications of the instrument are quite constant. 



