With the load tap open when we apply a signal with the generator the DC meter will read a voltage. Set the generator high enough to produce a good high reading but don't drive the circuit to hard or you will burn a resistor. If the reading drifts it could be an indicator of resistor heating. Now plug in a dummy resistor the same value as the bridge uses and the reading goes to Zero.
I hooked it to my short wire antenna and it responded to a change in the generator frequency. So I guess I need to build an antenna tuner. Putting a rheostat on the load terminal and adjusting it can give some feel for how the bridge functions.