Re: Flat conductors

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

Although the temperatures were not measured the estimated
delta(T) was about 80-25=55 C. At the room temperature
R was 2.62 kohms and it became 2 kohms at 80 C (see below).
Thus, the coefficient alpha must be close -0.004, most likely
between -0.003 and -0.005. The textbook value for carbon
is -0.0005. The difference (factor of ~8) is significant. How
can this be interpreted?

I hope that Barnard, who studied the dependence of R on
temperature, will describe the result of his observations. He
measured temperatures very accurately, up to 125 C, in
very small steps. Sending numbers to me is not as useful
as sharing them with the list. What is your alpha, Bernard?
Ludwik Kowalski wrote:

> 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