166 Prof. Faraday on the Use of Gntta Percha 



the best state: I will describe the qualities of a proper speci- 

 men, and reter to the differences afterwards. A good piece of 

 gutta percha will insulate as well as an equal piece of shell-lac, 

 whether it be in the form of sheet, or rod, or filament ; but 

 being tough and flexible when cold, as well as soft when hot, 

 it will serve better 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 

 induction, not being liable to fracture: in the form of thin 

 band or string it makes an excellent insulating suspender : 

 a piece 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 electrometers when they pass through 

 sheltering tubes, and larger plugs supply good insulating feet 

 for extemporary electrical arrangements : cylinders of it half 

 an inch or more in diameter have great stiffness, and form ex- 

 cellent insulating pillars. In these and in many other ways 

 its power as an insulator may be useful. 



Because of its good insulation it is also an excellent sub- 

 stance for the excitement of negative electricity. It is hardly 

 possible to take one of the soles sold by the shoemakers out 

 of paper or into the hand, without exciting it to such a de- 

 gree as to open the leaves of an electrometer one or more 

 inches; or if it be unelectrified, the slightest passage over the 

 hand or face, the clothes, or almost any other substance gives 

 it an electric state. Some of the gutta percha is sold in very 

 thin sheets, resembling in general appearance oiled silk ; and 

 if a strip of this be drawn through the finjjers, it is so electric 

 as to adhere to the hand or attract pieces of paper. The ap- 

 pearance is such as to suggest the making a thicker sheet of 

 the substance into a plate electrical machine for the production 

 of negative electricity. 



Then as to inductive action through the substance, a sheet 

 of it is soon converted into an excellent electrophorus; or it 

 may be coated and used in place of a Leyden jar; or in any 

 of the many other forms of apparatus dependent on inductive 

 action. 



I have said that all gutta percha is not in this good elec- 

 trical condition. With respect to that which is not so (and 

 which has constituted about one-half of that which, being ob- 

 tained at the shops, has passed through my hands), it has either 

 discharged an electrometer as a piece of paper or wood would 

 do, or it has made it collapse greatly by touching, yet has on 

 its removal been followed by a full opening of the leaves again : 

 the latter effect I have been able to trace and refer to a con- 

 ducting portion within the mass covered by a thin external 



