Art Unwin, KB9MZ wrote:
"Are you suggesting that the variometer---?"

I can`t say what Reg had in mind. What he wrote speaks for itself. The
change in mutual inductance between variometer coils causes a change in
their total inductance. As the sense of the rotatable coil can be
reversed, its inductance can be arranged to aid or oppose the inductance
of the fixed coil.

Terman says on page 20 of his 1955 edition:
"when two coils of inductance L1 and L2 , between which a mutual
inductance exists, are connected in series, the equivalent inductance of
the combination is L1 + L2 plus or minus 2M. The term 2M takes into
account the flux linkages in each coil due to the current in the other
coil. These mutual linkages may add to or subtract from the
self-linkages, depending upon the relative direction in which the
current passes through the two coils. Thus , when all linkages are in
the same direction, the total inductance of the series combination
excedes by 2M the sum of the individual inductances of the two coils."

I think Reg gave a reasonable answer. We may assume coupling (mutual
inductance) is high and that the coils are wound for equal inductances.

Now a short-cut. We know that inductance increases with the square of
the turns under common conditions. If we double the number of turns by
sensing the coils so they aid, we will quadruple the inductance, as Reg
said if I recall correctly.

Best regards, Richard Harrison, KB5WZI