Scales come in all shapes, sizes and configurations, but the basic component doing the measuring is nearly always a load cell.
A load cell is a kind of transducer, a device that converts one form of energy into another. Through load cells, digital scales change mechanical energy -- the smooshing or stretching caused by a sitting or hanging load -- into an electrical effect. The widely used strain gauge (you'll also see it as strain gage), for example, reads compression or tension as tiny changes in electrical resistance in a Wheatstone bridge [sources: Mashaney; Omega].
Let's break that down using a compression strain gauge as an example.
Compression occurs when an applied force reduces an object's volume, but it can also refer to a more general decrease in size along one or more dimensions. As it happens, squishing an electrically conductive material changes its electrical resistance, because longer and narrower wires are more resistant than shorter, wider ones [source: Craig]. Think of it like water pushing through a pipe: The longer and narrower the pipe, the harder it is to force water through it.
Various materials experience different resistance changes under deformation, a quality known as gauge factor. Gauge factor can also alter in response to temperature. Consequently, constantan alloy (55 percent copper and 45 percent nickel), which performs well at room temperature, has established itself as the go-to material for strain measurements [sources: Encyclopaedia Britannica; Craig; National Instruments; Pratt].
To pick up the change in resistance caused by weight compression, one or more strain gauges are placed within a Wheatstone bridge. A Wheatstone bridge is an electrical circuit that can detect an unknown electrical resistance by balancing it against known resistances elsewhere in the circuit. In a sense, it's like a balance scale for electrical resistance: The "weight" (resistance) on one side tells you the unknown "weight" (resistance) on the other [sources: Craig; National Instruments; Pratt].
A given bridge can contain 1-4 strain gauges. When multiple gauges are used, they're arranged in opposing directions to improve sensitivity and to mitigate temperature effects. Because the resistance change in a strain circuit can be minuscule, the signal often requires amplification [sources: Craig; National Instruments; Pratt].
As a load cell measures compressive resistance change, it transmits a signal to the CPU, which converts it into input for a display board, which then shows the result on a digital screen. This principle remains true whether you use a strain gauge or some other kind of measuring device.