Inductive Capacity of Dielectrics. 413 



there will be no displacement of electricity along bd when the 

 potential at b is 



and that at a is 



#i + k 2 k 3 + k 



That is, when k 1 k i =k 2 k 3 , b and d are at the same potential 

 and the telephone is silent. 



This method is only applicable to electrolytes that are poor 

 conductors. In case the electrolyte is a conductor, the telephone 

 will have no place of silence on account of the difference in 

 phase of the electrical waves in the two branches of the circuit. 

 But if the electrolyte conducts but slightly, silence can be 

 produced in the telephone by shunting a high resistance 

 around the other condenser in the same branch of the circuit 

 as that containing the electrolyte. In this experiment the 

 resistance employed consisted of two fine vertical glass tubes 

 connected at the bottom by means of a bottle, and the whole 

 filled with water. By means of wires that could be inserted 

 to different depths in the tubes, a wide range of resistance 

 can be obtained. The problem of the Wheatstone bridge using 

 an alternating current when the circuit contains resistance, 

 capacity, and self-induction has been worked out by Oberbeck*. 



The substances previously examined, when subjected to this 

 slowdy changing field having a period of about five hundredths 

 of a second, gave the following values for the specific inductive 

 capacity: — Castor oil 4" 65; olive oil 3*13; cotton-seed oil 

 3*09; petroleum 205; ebonite 2*55; quartz cut normal to 

 the optic axis 4*46 ; quartz cut parallel to the optic axis 4*38. 

 For purposes of comparison, the values of the dielectric con- 

 stants for rapidly and for slowly changing fields are arranged 

 side by side in the annexed table. 



Period, Period, 



Substance. 80,000,000 jer second. 20 per second. 



Castor oil 4*49 4'65 



Olive oil 3-02 3-13 



Cotton-seed oil 3"00 3"09 



Petroleum 1:99 2'05 



Ebonite * 2*32 2*55 



Quartz J_ to axis . . . 4'34 4*46 



Quartz || to axis ... 4*27 4*38 



The specimens of quartz used were cut from positive crystals 

 and the values for the specific inductive capacity in the two 

 directions were found to differ in the same sense as the index 



* Wied. Ann, xvii. p. 816 (1882). 



