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On the Use of Gutta Percha in Electrical Insulation. By 

 Michael Farday, F.R.S., Foreign Associate of the Aca- 

 demy of Sciences, &c. 



Mr Farady, in an interesting letter to Mr Phillips, pub- 

 lished in No. 214 of the Philosophical Magazine, March 

 1848, communicates the following curious particulars in re- 

 gard to Gutta Percha, which are quite new, and of import- 

 ance. " I have lately found," says Mr Farady, " gutta 

 percha very useful in electrical experiments ; and, therefore, 

 that others may take advantage of its properties, if they have 

 occasion, or are so inclined, give you this notice for insertion 

 in the Philosophical Magazine. Its use depends upon the 

 high insulating power which it possesses under ordinary con- 

 ditions, and the manner in which it keeps this power in states 

 of the atmosphere which make the surface of glass a good 

 conductor. All gutta percha is not, ho\vever, equally good 

 as it comes from the manufacturer's hands ; but it does not 

 seem difficult to bi*ing into the best state. I will describe 

 the qualities of a proper specimen, and refer to the differ- 

 ences afterwards. A good piece of gutta percha will insu- 

 late as well as an equal piece oP shell-lac, whether it be in 

 the form of sheet, a rod, or filament ; but being tough and 

 flexible when cold, as well as soft when hot, it will serve bet- 

 ter than shell-lac, in many cases, where the brittleness of 

 the latter is an inconvenience. Thus, it makes very good 

 handles for carriers of electricity in experiments on induc- 

 tion, not being liable to fracture ; in the form of thin band 

 or string, it makes an excellent insulating suspender ; apiece 

 of it in sheet makes a most convenient insulating basis for 

 anything placed on it. It forms excellent insulating plugs 

 for the stems of gold-leaf electi'ometers, when they pass 

 through sheltering tubes, and larger plugs supply good insu- 

 lating feet for extemporary electrical arrangements ; cylin- 

 ders of it, half an inch or more in diameter, have great stiff- 

 ness, and form excellent insulating pillars. In these, and in 

 many otlier ways, its [)o\ver as an insulator may be useful. 

 " Because of its good insulation, it is al-soan excellent sub" 



