484 Scientific Intelligence. 



6. On the Measurement of the Resistance of Liquids. — Two 

 methods for measuring the resistance of liquids have been pro- 

 posed, in both of which polarization is a minimum; one, the elec- 

 trometer method suggested by Lippmann,* the other the alter- 

 nating current method brought into use by Kohlrausch. Bouty 

 and Fotjssebeau have made comparative tests of these two 

 methods for the purpose of determining their relative value. In 

 the former, as modified by Foussereau,f the column of liquid is 

 included in a closed circuit, containing also a battery and a known 

 and adjustable metallic resistance. By means of a commutator, 

 the terminals of a condenser may be alternately connected with 

 two points in the liquid column or two in the metallic resistance, 

 the difference of potential of these terminals — corresponding to the 

 difference of potential of the points with which they are in contact 

 — being determined by means of a Lippmann electrometer in the 

 condenser circuit. By adjustment two points in the metallic cir- 

 cuit are found such that their difference of potential is exactly the 

 same as that between the two points in the liquid. The resist- 

 ance between the latter is then equal to that between the former, 

 which is of course known. Bouty in employing this method uses 

 a compensating battery in the condenser circuit, and has obtained 

 very satisfactory results with it. \ The experiments on the alter- 

 nating current method, were made by means of a small Deprez 

 generator revolving about 100 times per second. This current 

 passed through a Wheatstone bridge, a sensitive telephone being- 

 used in place of the galvanometer. Large electrodes, 0*01 square 

 meter each, were employed and the generator and resistances 

 were carefully insulated. But it was found next to impossible to 

 adjust the telephone to silence when metallic resistances formed 

 the sides of the bridge ; although the resistance coils used were 

 by approved makers. With low values, a minimum sound could 

 be distinguished ; but as the resistance increased, this minimum 

 sound became louder and less distinguishable. But, what is of 

 more importance, when the bridge was balanced for sound, it was 

 entirely unbalanced for the resistances; the error rising to 20 per 

 cent even. This result is manifestly due to the fact that these 

 resistance coils were not free from induction and the coefficient of 

 self-induction with alternating currents became a serious matter. 

 The sides of the bridge were then formed of liquid resistances. 

 Three of these consisted of pairs of glass jars containing zinc sul- 

 phate solution and amalgamated zinc plates ; a syphon between 

 them regulating the resistance of each pair. The fourth consisted 

 of a specially constructed liquid rheostat, made of two glass 

 cylinders, one above the other, containing each a copper electrode 

 of large surface immersed in copper sulphate solution. By means 

 of a glass tube passing through the bottom of the upper jar, 

 nearly to the bottom of the lower, communication is established 

 between them. A glass rod passing through the tube serves to 

 vary the liquid in the tube and so the resistance of the apparatus. 



* G. R., lxxxiii, 192, 1876. f J. Phys., II, iv, 189, May, 1885. 



% Ibid., II, i, 346, Aug., 1882, II, iii, 433, Aug., 1884. 



