Load cells may seem like high tech, but their roots lie in some truly classic Victorian scientific thinking.
Back in 1833, British scientist and mathematician Samuel Hunter Christie wanted to compare the resistance of wire in various thicknesses. However, his paper remain unknown until 1843, when another scientist, Charles Wheatstone proposed it in a paper for the Royal Society. His method of measuring resistance in electrical circuits became known as the Wheatstone bridge.
The Wheatstone bridge is a network of four resisters, one or more of which is an active sensor. By knowing the resistance of three resisters in the network (shown as R1, R2 and R3 on the diagram), you can measure the resistance on the unknown, shown as Rx.
This can be measured by the galvanometer in the middle (shown as VG) as follows.
When the ratio of the two resistances in R2/R1 is equal to the ratio of the two resistances in Rx/R3, the voltage between the midpoints (B and D) will be zero. In this case, no current will flow through the galvanometer. If the bridge is unbalanced, the direction of the current will shown whether R2 is too high or too low. You can then vary R2 until there is no current through the galvanometer, and the network is back in balance.
The idea is both simple and precise. When the Wheatstone bridge is used to measure a sample that is static and unchanging, the resistance ratios between the four legs remains the same. However, if the sample changes in any way, it will affect the resistance between the resisters, which can be measured. Wheatstone bridges are used in strain gauge measurements because the reading is very accurate and reliable.
Need the math(s)?
This detailed but not too techie video from MIT will help!
Need help with your Wheatstone bridge configurations?