Erik Malund said: for an IDEAL switch
and zero voltage and zero resistance when closed.
there ain't no such thing as ideal.

But i was asked to measure the CR of the Reed switch by passing 10mA current at 20mV through the contacts.
If this was possible, you would not need to measure the resistance, just calculate it (2 ohms).
You can't specify a voltage, the voltage is the drop across the switch AT a current. If you apply a voltage and the voltage is too high (20 mV may very well be too high) the switch is DEAD (melted)

The ONLY way to measure switch resistance is to pass a known current and measure the voltage drop.

Erik