[Test-Equipment] Question regarding antique LCR bridge

[Barry]

I have a Marconi TF1313 LCR bridge (designed in 1960).  When measuring L or C, the bridge is fed with an AC signal (1KHz or 10KHz).  The output goest to a couple of amplifiers and the detected signal is fed to a meter.  When the bridge approaches balance, the detected signal approaches zero and the meter indicates a null.  Standard stuff for a bridge.

When measuring R, though, a DC (full-wave rectified, unfiltered) signal is fed to the bridge and the output is connected to the input of the first amplifier through a chopper (old fashioned vibrator).  Amplification and detection is the same process as for L and C.

My question is why it is necessery to change the configuration of the bridge for R measurement?  Why wouldn't it work with an AC input for R the same as for L and C and eliminate the chopper?  Wouldn't a balanced bridge made entirely of R produce a minimum signal at the output and allow for null detection?

I know the answer is probably simple but I don't see why this is done this way.

Thanks,
Barry - N4BUQ