﻿568 Specific Inductive Capacities of Water, Alcohol, fyc. 



low speeds, however, the friction threw the readings out a 

 good deal. It then occurred to the writer to utilize the 

 suggestion made by Messrs, Swinburne and Kelly for electro- 

 static voltmeters, t. e. to immerse the instrument in oil. This 

 was done with good results. It was reasoned, then, that since 

 pure water insulates as well as indiarubber, and has, accord- 

 ing to the experimenters referred to above, a specific induc- 

 tive capacity of over 70, or 35 times that of oil, all difficulties 

 would be removed by its use. Water distilled in vacuo to 

 remove foreign gases was then tried, but gave no better 

 results than oil. The commutator was then taken off and 

 the movable cylinder suspended by a bifilar suspension, with 

 the result that in the case of oil and the alcohols the specific 

 inductive capacity came out very nearly equal to the square 

 of the refractive index, thus showing plainly that all the high 

 determinations hitherto given were erroneous. 



The reason of these errors was not far to seek. It lay in 

 the fact, pointed out by Maxwell and others, that electrolysis 

 gives a capacity effect. A number of determinations made 

 by students in my laboratory show that, for 133 periods per 

 second, and a current-density of *01 ampere per square centim., 

 each square centim. of electrode-surface has an apparent ca- 

 pacity of 400 microfarads when the electrolyte is caustic soda 

 and the plates nickel. It is for this reason that Kohlrausch's 

 method almost always gives erroneous results, as what is 

 measured is not the resistance of the electrolyte, but its im- 

 pedance. 



Since Messrs. Cohn and Arons, for example, used an 

 induction-coil to charge the plates of the electrometer im- 

 mersed in water, it follows that the voltage on the water- 

 immersed quadrants might easily have been 100 times that 

 on the other quadrant, and that the method is inapplicable. 

 All capacity measurements made by discharge are also in- 

 correct ; and the only correct method consists in purifying 

 the fluid till it no longer conducts appreciably, and then 

 measuring the attractive force between the plates when these 

 are charged from a continuous current source of high voltage. 

 Either a torsion or chemical balance maybe used. A certain 

 amount of leakage will always take place, but this must be 

 provided for by using a powerful source of current. 



It might also be mentioned that all the determinations of 

 specific capacity of substances such as sulphur &c. are in- 

 correct. For the reasons of this, those writers who have 

 treated of the capacity of laminated dielectrics may be 

 consulted. 



