In estimating the capacitance of a given configuration of conductors, comparison with known capacitances is often . consider two configurations of conduc- tors in which the (n-1) conductors held at zero potential are the same, but the one conductor whose capacitance we wish to know is different. in particular, let the conductor in one configuration have a closed surface s, and in the other configu. ration have surface si, with si totally inside s. (a) use the extremum principle of section 1.12 and the variational principle of problem 1.17 to prove that the capacitance c' of the conductor with surface s is less than or equal to the capacitance c of the conductor with surface s, that encloses s. (b) set upper and lower limits for the capacitance of a conducting cube of side a. compare your limits and also their average with the numerical value, c -0.655(4 € 4).