The physics of the process of the appearance of even harmonic components in the SHVTL with connected unloaded AT is due to the periodic change in the inductance of the magnetic shunt when the AC passes through it.
For this it is necessary that the input impedance of the line has a capacitive character and was approximately equal to the average value of the inductive resistance of the magnetic shunt of the autotransformer at this frequency.
If the deflection differs somewhat from this value, it is to be brought to 100 divisions by adjusting a magnetic shunt which varies the magnetic flux in the air gap of the milliammeter.
(5.) To avoid the liability of mistake, or of possible meddling by unauthorized persons, it was made necessary to use a small screwdriver to adjust the magnetic shunt.