244 PROCEEDINGS OP THE AMERICAN ACADEMY 



X7IL 



ON THE COMPARISON OF SOME ELECTRICAL 

 CONDENSERS. 



By Charles Nutt. 



Presented November 12, 1890. 



This article contains a comparative examination of some of the die- 

 lectrics most commonly used in condensers of large capacity. 



ParafRne paper condensers are by far the cheapest, and for this 

 reason it is desirable to know how good a condenser can be made 

 using this substance as a dielectric. Certainly very great differences 

 exist in paraffiue paper condensers, and these differences are often due 

 to the varying qualities of paper and paraffine used. 



There are several kinds of so-called " paraffine papers " in the 

 market, varying in thickness from tissue to heavy manilla. On ac- 

 count of small holes in it, two sheets of the tissue paper must be used 

 between consecutive sheets of tinfoil. 



One condenser made of this white tissue " paraffine paper," of about 

 2.5 microfarads' capacity, lost its entire charge in less than ten min- 

 utes. Its electrical resistance was less than a megohm. To be sure 

 that the fault was not due to a few bad sheets, I tested the whole 

 condenser by sections, all of which " leaked " badly. Other condensers 

 made of this paper and of the similar brown tissue " paraffine paper " 

 were as bad ; some were worse. 



Another condenser was very carefully made of the best quality of 

 commercial paraffine paper, — a strong white rag paper of good tex- 

 ture, probably often used in condensers. Its capacity, as measured by 

 discharging through the galvanometer immediately after charge, was 

 0.308 mfd. Its resistance, measured by an ordinary Wheatstone's 

 bridge, was only 740 ohms ! After loosening the clamps, the resist- 

 ance fell to 250 ohms. When tested by sections, the insulation of the 

 paper was shown to be uniformly poor. Tlie presence of one sheet of 

 this paper in a condenser would be enough to spoil it. 



My experience seems to show that commercial "paraffine paper" is 

 not fit for makmg condensers. 



In preparing paraffine paper for myself, it was necessary first to 

 decide upon the best kind of paper. I made a preliminary test, on 



