A spring scale measures distance that the object is displaced, whereas a beam scale balances the object against another mass. Both measure the mass of an object, although this is commonly referred to as the weight of an object.
Both types of scale rely on the force of gravity to operate. The beam scale balances weights placed at certain distances along a beam to equate the gravitational pull on the unknown mass. The mass attached to the spring scale is calculated from the gravitational pull, displacement and spring's elasticity.
A beam scale has the capability of measuring small to very large masses. It operates by moving attached weights along the beam until the arm is balanced with the object placed in the attached pan. According to John G. Webster, "along the beam, there are notched positions that are marked to correspond to the force applied by the sliding weights." The mass value corresponds to the notched positions. A typical example of a beam balance is one used in a doctor's office.
A spring scale is limited in its ability to measure mass. Because the spring has to be capable of withstanding the force placed on it by the weight of the object, it is limited by the strength capacity of its spring. The amount that the spring is displaced by the freely hanging object attached to one end corresponds to a certain mass. According to Webster, "when the force of gravity and the elastic force of the spring balance, the force is read from the scale, which has been calibrated in units of mass."
balance-beam image by Pali A from Fotolia.com