Well, the reason isn't anything to do with electrical properties.
The reason that we use four variable resistors here, we mount two
strain gages on the top, and two on the bottom, is because of thermal properties.
We want to negate the thermal expansion of this beam.
As it gets warm, the beam will elongate.
Well, if the beam elongates, so will those strain gages.
And that will change the resistance.
The things is, if these resistors, the top and bottom,
both elongate at the same amount, their ratios don't change, so
it still stays in a balanced Wheatstone configuration.
And that way, we negate all effects of thermal expansion.
And the only effects,
that we will be measuring with this, is the bending effects.
And that's what we were trying to measure.
Looking again, as I bend it this direction, it becomes positive voltage.
Bending it this direction becomes negative voltage.