830 Mr. K. T. Lattey on the Dielectric 



generally overlooked *. This may be because these ob- 

 servers have usually been somewhat guarded in their 

 statements. Drude 17 , for example, says: "Man wiirde 

 daher auf eine geringe Abnahme der Dielectricitats- 

 constante mit wachsendem Salzgehalt zu Schliessen haben. 

 Wenn audi diese Schlnss wegen der Grosse der Beo- 

 bachtungsfehler nicht mit voller Sicherheit vorlaufig zu 

 ziehen sein mag, so geht doch aus der Beobachtungen 

 zweifellos hervor, dass die Dielectricitatsconstante des 

 Wassers durch Auflosung eines Electrolyten selbst bis 

 zu der Leitfahigkeit K = 11.10 -7 jedensfalls nicht ver- 

 grossert wird." 



Practically all methods by which one can determine the 

 capacity of a condenser whose plates are separated by a 

 conducting medium involve the use of alternating currents ; 

 these may have a comparatively low frequency (2C0 to 

 300 radians per second, as in the experiments of Franke 28 , 

 Heerwagen 31,32 , and Rosa 49 ) or a considerably higher 

 frequencv (10 6 to 10 10 , as in the experiments of Niven 45 , 

 Harrington 30 , Nernst 41 - 44 , Marx 38 - 39 , Cole 4 , Drude 14 " 23 , 

 (Jolley n ' 12 , Lampa 35 , and many others). With moderate 

 frequencies (not exceeding 10 5 or 10 6 ) it is possible to 

 use forms of Wheatstone's bridge (see 1, 26, 27, 30, 33, 

 41-44, 48, 54, and 59). Silow's method of charging two 

 electrometers simultaneously by the same source of E.M.F. 

 and comparing their deflexions when each is filled with a 

 different dielectric, has been applied over a wide range of 

 frequencies (see 5-8, 2S, 31, 32, 46, 47, 49, 5.1, 52, 55, 57). 



It is not proposed to discuss either of these methods here, 

 but it may be pointed out that the results obtained from 

 them are only trustworthy when proper corrections are 

 made for the current passing through the condenser by 

 conduction. 



Imperfect insulators under the influence of an alternating 

 field may behave in one of two ways : (a) they may convey 

 a current in accordance with Ohm's law and dissipate the 

 power of the current at a rate proportional to i 2 r, or 

 (b) they may cause an absorption of electrical power 

 which is out of all proportion to the conductance they 

 display to a constant field or when only slow oscillations 

 are used. The second form of absorption has been studied 



* Lecher's observation 37 that a condenser had the same effect on 

 Ruhmkorf oscillations when it was filled with water as when it was 

 filled with 10 per cent, sulphuric acid has been quoted in favour of 

 the view that both these liquids have the same dielectric constant 

 < Palmer 46 ). It really shows that both behaved as conductors. 



