130 STATIC ELECTRICITY 



In the second case we have 



Dividing, we get 



They obtained the following values : 



K 



Distilled water 70 



E%1 alcohol 26'5 



Amyl alcohol 15 



Petroleum 204 



The remarkably high values for water and alcohol were con- 

 firmed by Rosa,* who measured the attraction between two plates 

 immersed in the liquid, connected through a commutator re- 

 spectively to the two terminals of a batten .supplying any desired 

 potential difference up to (50 volts. The commutator was rc\er-cd 

 from 2000 to 4000 times per minute, so that the charges of the 

 plates alternated rapidly. One of the plates was fixed, and the 

 other was suspended at the end of a torsion arm, the- small 

 torsion measuring the force. The specific inductive capacity 

 was determined by finding the ratio of the attraction for the 

 same difference of potential, with the liquid as medium and with 

 air. For the K medium the charges are K tim reat, 



and, therefore, the forces which are (p. 104) proportional to 



^ are also K times as great. Rosa obtained for water at 



25 K=75-7, and for alcohol at 25 K = 25'7. He found that 

 the conductivity of water might be increased many times by 

 adding minute quantities of acid without much change in the 

 attracting force. The force was, however, slightly 1 t -IK <1 by 

 the addition. 



Heerwagen,f using a method somewhat like that of Colin 

 and Arons, found the value of K for water and its variation 

 with temperature, the results agreeing very closely with the 

 formula 



K t = 80-878 -0-362 (f-17). 



NernstJ put the liquid to be tested in a conden>er and 

 determined the capacity by a Wheatstone bridge method, using 



* Phil. Mag., xxxi. (1891), p. 188. 



t Wild. Ann. (1893), xlviii. p. 35, and xlix. p. L7i ( . 



t Zeit. Phys. CAem. (1894), vol. xiv. p. 22. 



