# Formula for potential for 2 coaxial tubes

Could someone remind me of the formula for potential function for a system consisting of 2 coaxial tubes maintained at a fixed potential difference and the in between medium has uniform conductivity $c$? (I have forgotten how to calculate these things, have been a long time since high school!)

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Formula:$$V(r)=V_0\frac{(r-r_1)(r_1r_2)}{(r_2-r_1)(r_1r)}$$
I've not cancelled the $r_1$ on purpose, it makes the origin of the equation more apparent. –  Manishearth Feb 28 '12 at 16:40
And you can easily see that pd is $\propto \Delta\frac{1}{r}$ –  Manishearth Feb 28 '12 at 16:47