102 Mr. J. Swinburne on 



posing of course sufficient instrumental means. It seems 

 therefore that a specially sensitive electrometer would be 

 doubly effective here, as eliminating the shock and concussion 

 inevitably attendant on a high velocity and a large magnet, 

 which tend to conceal delicate observations. 



Hamburg, December 13, 1890. 



XII. Alternate Current-Condensers. By J '. Swinburne*. 



THOUGH condensers have been proposed for various 

 methods of distribution of electrical power, schemes 

 involving the use of them have never developed enough for 

 alternating current-condensers to be made commercially. It 

 is generally assumed that there is no difficulty in making 

 them ; all that is needed is to separate a number of thin plates 

 by means of sheets of insulating material. 



The first difficulty is insulation. If a condenser is to take 

 2000 effective volts, the insulation must be very good. Too 

 great thickness of dielectric cannot be allowed, as that would 

 lessen the capacity, and increase the cost of material. The 

 surface of such a condenser for, say, ten amperes is about a 

 thousand square feet, and the problem of making a thousand 

 square feet of thin insulating material that will safely stand 

 2000 volts is not easy. I have made samples which, though 

 only a little over a tenth of a millimetre thick, would stand 

 8000 effective volts for some time before breaking down. 

 When made up into a large condenser, two or three thousand 

 volts would generally break it down in a few hours. 



Insulation is not by any means the only difficulty, however ; 

 there is another, which is probably intimately connected with 

 it, and that is absorption of electric power in the dielectric, 

 and consequent heating. It is well known that Leyden jars 

 get hot if charged and discharged frequently ; and also that 

 the phenomenon of electric absorption is manifested. When 

 there is absorption, the current out of the condenser is not 

 proportional to the rate of decrease of the electromotive 

 force, and probably the current into it is not proportional to 

 the rate of increase. This means that the condenser absorbs 

 energy and converts it into heat. 



Such questions have been frequently studied in connexion 

 with curves of sines, but it is easier to show the truth of this 

 generally. Let C be the current, E the electromotive force, 



* Communicated by the Physical Society : read December 12, 1890. 



