"Art Unwin wrote
In general terms.
Can we say that the variance
of an inductance containing a variometer
is zero to twice the inductance of the rotatable?
The variometer is not talked about much these days
so it is difficult to determine if something
else is at play other than what apears obvious.
I use such a set up in a coupling situation
together with variable physical separation to
prevent kickback and interference from tail
end unused inductance
I am not knoweledgeable enough to understand
all the possible implications so the variometer
seems the best place to start.

====================================

The inductance of the two identical coils in series is FOUR times the
inductance of a single coil provided the coupling coefficient between the
two coils approaches unity. It never does.

The main disadvantage of a variometer in a tuned circuit is that as the
mutual coupling and resulting inductance is reduced then the amount of wire
in use remains constant. So the loss resistance remains relatively
constant. And so a VERY low Q occurs at small values of inductance.

No good when used in antenna tuners. Which is great pity. The motion of one
coil rotating inside the other is beautiful to see.
----
Reg, G4FGQ