Weight is the resultant force acting on a mass (in a vacuum) due to the Earth's gravitational field corrected for the effect of the Earth's rotation. Units of weight are based upon an acceleration of gravity. When the weight of an unknown is determined by comparison with a known weight, there is no error in the readings due to gravity variations. The varying buoyant effect of the atmosphere is negligible when the density of the unknown is approximately the same as that of the standard. In precision weighing, the buoyant effect of air must be considered.
The equal-arm balance is probably the most common form of instrument for measuring weight. These balances are made in many designs and sizes; in some the knife-edge fulcrums are replaced with flexure plates; others have arms of unequal length. Conventionally, the unknown weight is placed on one pan, the known weight on the other. The final securing of a balance is done by adjusting the position of a rider, or small weight, on a bar of the balance arm bridge. The condition of balance is indicated when the pointer swings equal distances from its rest point.
The mechanical-type industrial scale incorporates a number of levers with precisely located fulcrums to permit heavy objects to be balanced (weighed) with small, convenient counterweights or counterpoises.
The pendulum-type mechanical scale balances the force of the load by the rotation of a bent lever. With this construction, the deflection of the load on the scale moves the counterweights through the lever system so that their center of gravity is at a greater distance from the final fulcrum. Thus the increased lever arm of the counterweights automatically balances the load.
The spring scale utilizes the deflection of a spring to measure the load. If sensitive enough, such a scale can detect and indicate the effect of differences in the weight of a body due to changes in elevation.
In hydraulic systems, the load applied to the load cell piston is converted to hydraulic pressure. The effective area of the piston must be known. The pressure may be measured at a remote point by a pressure-gage, such as a Bourdon tube.
Pneumatic systems detect the load by a sensitive nozzle and flapper system and balance the load by modulating an air pressure in an opposing capsule.
Electrical weighing systems usually involve the electrical measurement of the elastic deformation of a mechanical element under stress. The strain gage is attached to the weighing element in a manner to produce the maximum resistance change per unit of load. The change in resistance with load is measured and amplified by electronic means, and the load is read on a potentiometer.