Variometers are variable inductance coils.
There are two solenoids, one rotating inside the other.
Maximum inductance occurs when the two coils are in phase.
Minimum inductance when in antiphase.
Advantage - no switches.

DO NOT USE IN TRANSMITTER HIGH POWER CIRCUITS !

The whole length of wire is in circuit at all settings of the variometer.

Consequently as the inductance decreases the wire resistance remains
constant.

So coil Q falls to very low values at small inductance values. i.e., at the
high frequency end of the tuning range - where higher losses and an
increase in temperature are already at their worst.

If the two coils are of similar sizes then Q can fall to near zero at low
inductance settings.

Some variometers have the inside coil much smaller than the other. The
inductance range is relatively small. The reduction in Q may be acceptable
for low power applications and receiving purposes.

But the motion of the two coils, one inside the other, built into a working
transmitter would fascinate visitors to the shack. An attraction quite
capabable of overcoming the disadvantage of a ridiculous low Q at 30 MHz.
It's even better than watching one set of 500pF capacitor plates slowly
disappearing inside the other.
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Reg.