|| This is the basic 4-corner Wheatstone Bridge circuit.
It is a voltage divider where E is an applied voltage across two opposite
corners and eo is the voltage read by an instrument across the
other two corners. If all four resistances are equal, the voltage difference,
eo, will be zero.
|| This is a two-wire strain gage bridge circuit. Here
a strain gage is placed in the circuit as one of the resistances. As the
gage is strained, its resistance will change, and hence cause a voltage
difference eo to exist. This voltage is proportional to the
resistance change and the strain. Note that the lead wires to the gage
also have resistances, RL. The strain measurement will include
their resistances and introduce an appreciable (>1%) error if they are
long (>20 ft.) Also any thermal strains in the wires due to temperature
changes will be included in the strain indicator's output.
|| The three-wire strain gage bridge circuit neutralizes
the lead wire and temperature effects. In this case the third wire is used
to "move" one corner of the bridge to the gage. Hence, one lead wire resistance
is added to one arm of the bridge and the other to another arm. Thus, the
length and temperature effects add the same amount to each side of the
bridge and effectively cancel. The third wire (in the center of the diagram)
is attached to the voltage meter and has no significant effect on the reading.