Strain gages are at the heart of most modern load cells. A strain gage (also spelled strain gauge) is a device whose electrical characteristics change as it's either compressed or placed in tension. This is generally a change in electrical resistance (although capacitive and inductive strain gages exist, they are less convenient to use, often frail, and generally more prone to error). Strain gauges are manufactured from thin metal wires, which are attached to a surface on a load cell such that a change in the shape of the load cell (due to an external force) will cause the wire to be shortened or lengthened, altering the wire's resistance. Many load cells contain four strain gages, with two in tension, and two in compression. This provides maximum sensitivity and temperature compensation.
The changes in resistance that occur within a strain gauge are very small, and sensitive circuits must be used to detect these changes accurately. The fundamental electrical circuit used with strain gages is called a Wheatstone Bridge. The Wheatstone Bridge was actually invented by Samuel Christie, but Sir Charles Wheatstone popularized it, and the device eventually was named after him. It is a set of four resistors connected in a circuit.