Tests of electric strength of dielectrics 253 



Thick wax 3 less than F. 



2 nd rosin, if less, plate air 5 J more, and I st made rosin same as F. 



N.B. The I st made rosin was made of the same proportion of rosin and 

 bees wax as the others, but not of the same parcel : it is uncertain how much 

 it was heated in making the mixture. 



Result of the exper. on plate air. 



520] Breaking of electricity through thin plates of lac, exper. rosin and dephleg. 

 bees wax. 



Thin plates were pressed out of lac, experimental rosin and dephlegmated 

 bees wax, very thin at one end and thicker at the other. The tinfoil was stript 

 from one side of these plates but the other left on, and was fastened to a piece 

 of glass with gum water, and a piece of tinfoil fastened to the under side of glass 

 communicating with the other. 



These plates were placed on [the] negative side of the machine with wire 8 

 bearing against bottom and a flat piece of brass at top on which wire j8 was 

 suffered to rest. The machine was electrified in usual degree, and the bit of 

 brass shifted from thicker to thinner part, till the electricity broke through the 

 plate and discharged the jars. 



A piece of the plate with the tinfoil under it was then cut out of the size of 

 the brass plate, as near as possible to the place where the electricity broke 

 through, and the thickness of the plate found by weighing it and also the 

 tinfoil after the plate was separated from it. 



The thickness of the plates thus found was as follows f, the specific gravity 



being 

 \ rosin > , ?! 



supposed 

 Uac li- 



I" wax 

 J. rosin 



955 

 i -06 



wax at I st place -0130 

 2nd -0123 

 rosin -0131 



lac '0143 



* [See Art. 343. The inches of electricity are circular inches, and to reduce 

 them to globular inches must be multiplied by 12-1, the diameter of globe, and 

 divided by 18-8, the diameter of a circle which has the same charge. The com- 

 puted power here is the square of the diameter divided by the thickness, and this 

 must be multiplied by 8 to get the computed power as denned in Art. 311.] 



f [With the "usual degree of electrification" Lane's electrometer discharged at 

 04 inch. See Art. 329. The electric strength of wax, rosin, and lac is therefore 

 about three times that of air.] 



