The kinetic energy

Now

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with similar expressions for G and H.

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with similar expressions for and . Since the particles are supposed to be very small, we shall neglect those terms in F which depend on *a²* and *a'²*.

The part of the kinetic energy we are concerned with involves the product *ee':* let us first calculate that part of it arising from the product of that part of F due to *e* with that part of due to e'. We shall take the line joining the particle as the axis of *x*; and for brevity we shall denote by σ.

The coefficient of *uu'* in the part of the kinetic energy we are considering

Now, for values of *r* > R,

;
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Now, since

where Q_{n} is a zonal harmonic of the nth order; and since the product of two harmonics of different degrees integrated over