Charge sharing between touching conducting spheres

For externally tangent conducting spheres of radii $a,b$, centre separation $D=a+b$, and common electric potential $V$, the method of images givesAlternating spherical image reflections cancel the nonconstant boundary electric potential. Pairing terms makes each series convergent. The ratio is generally not $a:b$: that rule uses isolated capacitances and applies to widely separated conducting spheres connected by a wire. For $b=2a$, the smaller sphere carries the fraction $1/2-\pi /(6\sqrt{3}\ln 3)\simeq 0.22483$ of the total electric charge.