MacCullagh's formula

MacCullagh's formula

[mə′kəl·əz ‚fȯr·myə·lə]
(physics)
A formula for the potential due to a distribution of mass or charge at an external point: the potential V at a point P resulting from a distribution of mass or positive charge centered about a point O is V = (kM/r) + (k /2 r 3)(A + B + C- 3 I) + O (1/ r 4), where r is the distance from O to P, k is the gravitational or electrostatic constant, M is the total mass or charge, A, B, and C are the principal moments about O, I is the moment about OP, and O (1/ r 4) is a quantity that falls off at least as rapidly as 1/ r 4.