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.
Time
of
Relative Resistance Relative Resistance
Electrification. at
0º C (32º
F.) at 24º C (75º
F.)
1
minute
100
5.51
2
"
127.9
6.
5
"
163.1
6.66
10
"
190.9
6.94
20
"
230.8
7.38
30
"
250.6
7.44
60
"
290.4
7.6
90
"
318.3
7.66
In cable testing one minute is generally taken as the time of
electrification.
Pressure increases the resistance by the formula Rp=R (1+ .00327 P) in
which Rp is the resistance at pressure p--R resistance at atmospheric
pressure--p pressure in atmospheres. Thus in the ocean at a depth of
4,000 meters (2.4855 miles), the resistance is more than doubled. The
longer the pressure is applied, the greater is the resistance.
The specific inductive capacity of gutta percha is 4.2.
Good gutta percha should not break when struck with a hammer, should
recover its shape slowly, and it should support much more than 300 times
its own weight.