The very old, but very
famous Wheatstone bridge is an electrical circuit that has been widely used in
many applications all around the world. Originally invented by Samuel
Christie in 1833 as a means of comparing the resistance of wires and metals,
the invention was eventually popularized by Charles Wheatstone in 1843.
Wheatstone provided a mathematical formula that displayed the relationship of
currents, voltages, and resistances by applying the principles of Ohm's
law. The Wheatstone bridge is frequently used in resistance measuring
applications, such applications include resistance thermometer, strain gauge,
and even soil analysis.

Let's say for example
we are trying to measure the resistance of a soil type. Of course the
soil's resistance value is variable, as the resistance changes when the soil is
watered. For experimental purposes, we decide we want to measure the
resistance when the soil is dry, and also when the soil is moist after
watering. The circuit of the Wheatstone bridge is displayed in Figure 1, note how
there are a total of 4 resistors, with two branches in parallel, where each
branch has two resistors in series. Three of the resistors are usually
known, while the other is unknown. In this case, the unknown resistor
element is soil that is squeezed between two electrodes.

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