To get the general axial multipole moments, we replace the point charge of the previous section with an infinitesimal charge element λ λ ( ζ ζ ) d ζ ζ , where λ λ ( ζ ζ ) represents the charge density at position z = ζ ζ on the z -axis. If the radius r of the observation point P is greater than the largest | ζ ζ | for which λ λ ( ζ ζ ) is significant (denoted ζ ζ max ), the electric potential may be written Φ Φ ( r ) = 1 4 π π ε ε ∑ ∑ k = 0 ∞ ∞ M k ( 1 r k + 1 ) P k ( cos ⁡ ⁡ θ θ ) where the axial multipole moments M k are defined M k ≡ ≡ ∫ ∫ d ζ ζ λ λ ( ζ ζ ) ζ ζ k