[Mondial]

I am not sure why they added the resistor other than to reduce the Q of the circuit and the relative sharpness of the null. I did notice as I increased the capacitance over the resonant value, the higher harmonics increased due to the increased capacitive coupling present. Perhaps the resistor reduces this effect.

In any case, the extra resistor only adds to the series resistance of the field coil itself already present with respect to the RLC circuit formed.