246 PROCEEDINGS OF THE AMERICAN ACADEMY 



The rag and the pulp papers show no disposition to change in resist- 

 ance while the current is passing, and they give decidedly the best 

 insulation. The bond and the linen papers seem to have constantly 

 changing resistances. The newspaper cannot be obtained free from 

 holes and bits of metal. The rag paper, on the other hand, is of good 

 texture, and admirably suited for our purpose. 



The galvanometer used in the above tests is a form of ballistic 

 galvanometer, devised by Professor B. O. Peirce of Harvard College. 

 The coils have a resistance of 2764 ohms. The period of the needle 

 is 53 seconds. 



A deflection of one scale division (equal to a deflection of 10 milli- 

 meters with a scale distance of 2.8 meters) indicates a current of 

 8. X 10~^ amperes. With four gravity cells, one division indicates 

 the presence in the circuit of 500 megohms. The resistance of the 

 paper may therefore be expressed in megohms. 



Construction of Condensers. 



The paraffine was taken from 15 lb. cakes. Experience showed 

 that, while dipping the paper, care must be taken to keep out dust and 

 dirt, especially coal-dust, and to prevent the parafiine from getting hot 

 enough to smoke. Each sheet was examined before and after dipping 

 for specks of dust and metal and for holes. Each was dipped, allowed 

 to soak, and then cooled by itself. 



The condensers are numbered for convenience, and briefly described. 



Condenser No. 1 was made of sheets of bond paper dipped in the 

 paraffine, with sheets of tinfoil between them. It was tested for leak- 

 age during the process of building and after it was clamped. The 

 clamps in this and the other condensers to be described were strong 

 oak boards of suitable size, bolted together so as to get a large and 

 evenly exerted pressure on the condenser. The terminals of tinfoil 

 at each end were wrapped about a thin strip of copper, from which a 

 wire passed to the outside to make connections, and were held by a 

 strip of board screwed to the lower clamp. These clamps, etc. were 

 carefully insulated from the condenser. All of the condensers were 

 built on a similar plan, — parallel plate condensers with half the tin- 

 foil connected at one end, the alternate sheets making the other half 

 at the other end. 



Dimensions of No. 1. 



Sheets of paper 80 



Thickness of condenser 1.14 cm. 



of tinfoil 05 " 



