Letter from [William Thomson, Lord] Kelvin, Netherhall, Largs, Ayrshire, to [Joseph] Larmor Larmor will see how mutual action between two rings with vortical motion through each is the same but in contrary direction to the force between the two electromagnets. A constant current is maintained through a circuit of wire equal and similar to the ring. Kelvin expands upon this, giving equations for the kinetic energy in the fluid motion. He concludes by asking if there is anything that Larmor does not understand and he will explain it if he can, referring to one of his 'Popular Lectures'. 26 December 1893