The more common method of measuring force, however, is by _distortion_, that is, by measuring the change of shape of a body caused by the force. In doing this, use is made of Hooke's Law (Art. 32), in which it is stated that "within the limits of perfect elasticity," changes of size or shape are directly proportional to the forces employed. That is, twice as great a force will produce twice as great a change of shape and so on. [Illustration: FIG. 55.--A spring balance.] A common contrivance using this principle is the spring balance (Fig. 55), with which all are familiar, as ice scales, meat scales, postal scales, etc. The object which changes shape in this device is a coiled spring contained in the case of the instrument. The balance is so constructed that when the spring is pulled out as far as possible it has not reached its limit of elasticity, since, if the spring were stretched so as to exceed its elastic limit, the index would not return to its first position on removing the load. (See Arts. 30-32.) =73. Graphic Representation of Forces.=--A force is said to have three elements. These are (a) _its point_ of _application_, (b) _its direction_, and (c) _its magnitude_. For example, if there is hung upon the hook of a spring balance a weight of 5 lbs., then we have: (a) its point of application on the hook of the balance, (b) its downward direction and (c) its magnitude, or 5 lbs. These three elements may be represented by a line. Thus in Fig. 56_a_, a line _AB_ is drawn as shown, five units long; _A_ represents the point of application; _B_, the arrow head, shows the direction; and the length of the line (five units) shows the magnitude of the force.