I have built up this circuit as in the attached image, and I measured the peak-peak voltage for open circuit and closed circuit using CRO.

I need to calculate Rm.
I have worked out that Rm = (VRs)/(e-V).

But I don't know how to get 'e' and 'V'...

 

You've already got them. Sigma is the source voltage which you measued with the switch closed, and V is the voltage across Rm which you measured with the switch open.

 

Thanks a lot!

I have one more question...
For the circuit in the attachment, how do I verify that the expected value of the resistance in the signal generator (Rth) must be small compared with the expected impedances of Cext and C1 ?