Introduction
All scales measure weight by recording the movement or change in shape of a calibrated component, whether it is mechanical (spring or beam) or electronic (force sensor). All scales have a maximum capacity, as well as a degree of precision and accuracy. A typical scale for a doctor's office might be a beam scale, with a set of weights across a beam at the top, but these are generally too large and expensive for home use.
Mechanical Scales
The simplest type of bathroom scale is a spring scale. The platform of the scale is evenly supported by a set of levers which attach to a short spring. The spring is then connected to an output component. The output component may be a gear beneath a dial which moves a pointer to indicate weight.
A mechanical scale can also be a hybrid, where the change in spring length is channeled into a rheostat or other electronic component which converts the length change into a voltage change. This type of mechanical scale will have a battery and a digital read out.
Electronic Scales
In a fully electronic scale, force transducers beneath the platform convert a change in force into a change in voltage. When you turn the scale on, the battery is sending a particular voltage through the circuits of the scale. Weight applied to the force transducers will alter this voltage and convert to a weight read out.
The use of force transducers removes the lever and spring mechanics from the scale and allows the top platform of the scale to be made of any material which will not deform. Therefore, fully electronic scales can have metal, glass, or ceramic platforms, and be thin in overall profile.
Hybrid and Electronic Scale Enhancements
Because there is already a power source for the scale, digital mechanical scales and fully electronic scales can have additional features, such as the ability to store weight data. If contact points are built into the platform of the scale, the available power also allows for estimating body composition. These contacts send a tiny current through the body and measure resistance. The current flows easily through water--fluid within and between cells in muscle--but slows down through fat. The resistance number is related to total body water content, and a chip within the scale will convert the data into an estimate of overall lean body mass and percentage body fat.
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