> We must be thinking of two different things > here. I don't see how opening the contacts > wouldn't interupt current through the field > coils.
The diode is in parallel with the field coil and reverse-biased when the contacts are closed. It is the nature of inductors to resist sudden changes in current (v=Ldi/dt). When the contacts open in the absence of a diode or snubber the current continues to flow (though decaying rapidly). It quickly charges the stray capacitance across the contacts to a high enough voltage to arc over. The diode shorts out this voltage by allowing the current to flow through it. The current then decays at a rate determined by the RL time constant of the inductance and resistance of the coil. A resistor in series with the diode will make it decay faster at the price of higher voltage across the contacts.
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