248 PKOCEEDINGS OF THE AMERICAN ACADEMY 



of only 15,000 ohms, and an electromotive force of its own large 

 enough to deflect the galvanometer "off scale" I By charging No. 72 

 with this electromotive force, I found it to be ^:^ volt. Of course such 

 an instrument is useless as a condenser. 



Condenser No. 9 was made of sheets of hard rubber carefully 

 cleaned and laid up. Paraffine paper was used to insulate the co;i- 

 denser itself. 



Dimensions op No. 9. 



Mean effective area of tinfoil (12 6 X 24 cm.) 302.4 sq.cm. 



Number of sheets 179 



Area of both surfaces of one set of tinfoil sheets . . . 44,754 sq.cm. 



Mean thickness of condenser 4.17 cm. 



" " tinfoil sheets 098 " 



" " rubber " 4.072 " 



" " single rubber sheet 027 " 



Calculated capacity of equal air condenser 165 mfd. 



Mean observed capacity 389 " 



Specific inductive capacity 2.35 



Gordon gives as a mean value for the specific inductive capacity of 

 ebonite 2.28, though some of his specimens yielded the value 2.31 ; 

 Schiller gives 2.76; WUllner gives 2.56. 



Condenser No. 10 was made of about 80 sheets of thin naked mica. 

 The margin around the tinfoil was only about a centimeter. 



Condenser No. 11 was made of 32 mica sheets (15.4 X 20.5 cm.), 

 coated, for insulation, with pure shellac applied with a small brush. 



Condenser No. 12 was made of 40 sheets of the same lot of mica 

 used in No. 11, coated, for insulation, with paraffine. 



Condenser No. 13 was made of 20 sheets of this same lot of mica, 

 coated, for msulation, by dipping into a dilute solution of the shellac, 

 and dried a week before they were used. 



Condenser No. 72* is a mica condenser made by Elliott Brothers, 

 London. Paraffine is used for insulation. 



With these twenty condensers I have made a large number of obser- 

 vations, some of which will be found in the following tables. The first 

 table shows the degree of insulation attained. The condenser was 

 charged, then immediately discharged, and the reading recorded in the 

 second column. It was then charged again, and disconnected for the 

 time noted in the first column. The third column gives the throw of 

 the needle at discharge, and the fourth the apparent percentage of 

 loss. 



* See these Proceedings, vol. iv. p. 145, " On the Charging of Condensers 

 by Galvanic Batteries," by B. O Peirce and R. W. Willson. 



