Gutta Percha.
The hardened milky juice of a tree, the Isonandra gutta, growing in
Malacca and other parts of the Eastern Archipelago. It is much used as
an insulator or constituent of insulators.
Resistance after several minutes electrification per 1 centimeter cube
at 54º C. (75º F.), 4.50E14 ohms.
The specific resistance varies--from 2.5E13 to 5.0E14 ohms. A usual
specification is 2.0E14 ohms. The influence of temperature on its
resistance is given in Clark & Bright's empirical formula, R = R0 at, in
which R is the resistance at temperature tº C--Ro the resistance at 0º C
(32º F), a is the coefficient .8944.
The resistance increases with the time of passage of the current, the
variation being less the higher the temperature.