In order that the separate strips and layers of the spring so made, figs. 174 and 175, may not slip out of place, the strips _a a_, _b b_, etc., are made in one piece, and all the plates are inclosed with a strap, _F_, figs. 176 and 177. The plates, instead of being cut from a piece like that represented in fig. 173, are, however, made out of steel of the proper width, and the ends, instead of being cut off pointed as represented, are sometimes drawn out thinner on the ends, like the point of a chisel, or oftener still cut off straight, as shown in fig. 177. [Illustration: _Fig. 176._ _Fig. 177._ _Fig. 178._ Scale ³⁄₄ in. = 1 foot.] In order to hold the plates together a band, _F_, is put around the middle. This is put on hot, and becomes tight by contracting as it cools. The centre of the spring has a hole drilled through it with a pin, _s_, fig. 178 (which shows a cross section of a spring), to prevent the plates from sliding endwise. The plates at each end usually have a depression, _a_, fig. 179 (which is a cross section of a plate on a larger scale than the preceding figure), made in them on one side, and a corresponding elevation, _b_, on the other. The elevation on one plate fits into the depression on the other, and thus prevents the plates from slipping sideways. [Illustration: _Fig. 179._ Scale 6 in. = 1 foot.] [Illustration: _Fig. 180._ Scale ³⁄₄ in. = 1 foot.] QUESTION 286. _How should springs be curved?_ _Answer._ Springs should be curved so that when they bear the greatest load which they must carry they will be straight. If they are curved too much they are subjected not only to a strain which bends the plates, but to one which has a tendency to compress them endwise. Thus if a spring like that represented in fig. 180 is bent into a half-circle, it is obvious that the strain at the ends has no tendency at all to bend the plates, but only to compress them endwise. Near the middle the strain will of course bend the spring. In the one direction the spring is flexible and elastic, and in the other it is not; and as the strain of compression depends on the amount of curvature, the greater the latter is, the less flexibility and elasticity the spring will have. Springs are often given a double curve, as shown in fig. 181. This is not to be recommended, because when a spring bends the plates must slide on each other. If they have but a single curve, they will do so and remain in contact through their whole length, but if they have two curves they will separate and therefore “gape,” as it is called. [Illustration: _Fig. 181._ Scale ³⁄₄ in. = 1 foot.] QUESTION 287. _What is meant by the elasticity of a spring?_