The temperature coefficient of resistivity for carbon is known
to be negative. Pasco paper can be used to demonstrate this.
Here are my data showing that R (V/mA) goes down when
the temperature increases.
volts mA temp
============================
50 19.1 room
75 30.0 slightly warmer
100 42.5 clearly warmer
150 70.5 very hot
200 99.9 too hot to hold
As you can see, the temperatures were not measured. I waited
several minute after each change of voltage (till the current
stopped changing). I was afraid that the sheet might burst
above 200 V. It would not be hard to measure T but I did
not do it. Worth sharing? I hope so. My resistor was defined
by two concentric circles silver-painted on Pasco paper. The
radii were 1.5 cm and 6 cm. I will tell you more about this
circular geometry in another message.
Ludwik Kowalski