lic compounds are
alloys of iron, manganese, nickel and copper, some of which were
discovered by Edward Weston, in the United States. One well-known alloy
of copper, manganese and nickel, now called manganin, which was brought
to the notice of electricians by the careful investigations made at the
Berlin Physikalisch-Technische Reichsanstalt, is characterized by having
a zero temperature coefficient at or about a certain temperature in the
neighbourhood of 15 deg. C. Hence within a certain range of temperature
on either side of this critical value the resistivity of manganin is
hardly affected at all by temperature. Similar alloys can be produced
from copper and ferro-manganese. An alloy formed of 80% copper and 20%
manganese in an annealed condition has a nearly zero
temperature-variation of resistance between 20 deg. C. and 100 deg. C.
In the case of non-metals the action of temperature is generally to
diminish the resistivity as temperature rises, though this is not
universally so. The interesting observation has been recorded by J. W.
Howell, that "treated" carbon filaments and graphite are substances
which have a minimum resistance corresponding to a certain temperature
approaching red heat (_Electrician_, vol. xxxviii. p. 835). At and
beyond this temperature increased heating appears to increase their
resistivity; this phenomenon may, however, be accompanied by a molecular
change and not be a true temperature variation. In the case of
dielectric conductors and of electrolytes, the action of rising
temperature is to reduce resistivity. Many of the so-called insulators,
such as mica, ebonite, indiarubber, and the insulating oils, paraffin,
&c., decrease in resistivity with great rapidity as the temperature
rises. With guttapercha a rise in temperature from 0 deg. C. to 24 deg.
C. is sufficient to reduce the resistivity of one-twentieth part of its
value at 0 deg. C., and the resistivity of flint glass at 140 deg. C. is
only one-hundredth of what it is at 60 deg. C.

TABLE VIII.--_Electric Volume-Resistivity of Dielectrics reckoned in
Millions of Megohms (Mega-megohms) per Centimetre-cube, and in Megohms
per Quadrant-cube, i.e. a Cube whose Side is 10^9 cms._

+------------------------+---------------------------+----------+
| | Resistivity. | |
| +------------+--------------+ Tempera- |
| Substance. |Mega-megohms| Megohms