Bathroom scales utilize a series of levers to distribute weight evenly while placing tension on a spring connected to a dial. The spring is calibrated so that it elongates under force, turning the attached dial until it displays a number proportionate to the weight distributed by each lever.
Unlike conventional scales which make use of a series of counterweights, most bathroom scales rely on a simpler spring and lever design, which allows for smaller and lighter construction. Most scales feature a four-lever arrangement that is designed to function simultaneously. This ensures that weight is transferred to the spring evenly so that it produces an accurate measurement. By measuring only a proportionate fraction of weight, scales are able to make use of much smaller springs in their construction.
Digital scales operate by converting mechanical energy into an electrical effect through a device known as a load cell. These devices often make use of electrically conductive materials which are compressed in order to produce a variation of their level of electrical resistance. To measure changes in electrical resistance, digital scales make use of a device known as a Wheatstone bridge, which detects electrical resistance by measuring it against the level of resistance found elsewhere within the circuit.