268 PROCEEDINGS OF THE AMERICAN ACADEMY 



Condenser No. 72. 



i:Z^. Throw,. Ttacla 



.002 67.84 3.0 71.0 



.026 08.30 4.0 71.0 



.226 00.30 5.0 71.3 



.840 70.48 10.0 71.16 



1.0 70.80 15.0 71.6 



2.0 71.0 80.0 715 



These results indicate that, while hard rubber is probably the best 

 dielectric for a standard microfarad, the paraiFine paper condensers, 

 Nos. 3, 14, and li), and the standard, No. 72, are extremely well in- 

 sulated and vary very little with time of charging. The experiments 

 prove that a parailiue paper condenser can be made that compares 

 favorably with a good mica condenser. 



The mica condensers that I made ** leaked " more than No. 72, be- 

 cause, I suspect, of the narrowness of the margin of mica around the 

 tinfoil. Tlie paraffine paper condensers gave larger residual dis- 

 charges than either the mica or hard rubber condensers. That was 

 their worst fault. 



A paratfine paper condenser is necessarily much larger and heavier 

 than a micA condenser of the same capacity, but far less expensive. 

 In the tables below I have estimated the cost of the materials in 

 microfarad condensers, using the three sorts of dielectrics that I found 

 best adapted for making condensers. 



Hard rubber $30.00 



Tinfoil 1.50 



Clamps 75 



$32.25 



Mica $10.00 



Tinfoil 50 



Shellac 25 



Clamps 76 



$11.60 



Paraffine $.75 



Paper 50 



Tmfoil 76 



Clamps 76 



$2:76 



Jefferson Phtsical Laboratory. 

 June, 1890. 



