Spring-balances with helical springs are applied to several useful purposes besides that of the weight of bodies. A spring of this character is sometimes used to hold down the lever of the safety-valve in a steam-engine boiler, the movement of the index also showing the pressure of the steam. Such an apparatus is useful in a locomotive engine, the shaking motion of which might derange a valve loaded with moveable weights. A helical spring balance forms also a good cable-stopper. When applied to the measure ment of muscular force, the tractive power of a locomotive carriage, &c., one end of the cylinder in which the spring is enclosed is made fast to an immoveable object, and the power to be measured is applied to the sliding-rod. If used to ascertain the force necessary to draw a carriage, the spring is placed between the carriage to be drawn and the power employed to draw it. In using a spring-dynamometer for this purpose, especially when the carriage is moved by animal power, some inconvenience is occasioned by the vibration of the index with every trifling variation in the force applied, to remedy which Mr. H. R. Palmer contrived an apparatus in which the quick vibration of the spring is checked by means of a piston moving in a cylinder filled with oil. A very narrow space is allowed for the oil to pass between the edge of the piston and the cylinder, so that a considerable resistance is opposed to the motion of the and the springs, and the index consequently represents the mean amount of force applied without being affected by sudden variations.
The ingenious method adopted by Mr. Martin for transmitting the motion of a spring to an index moving upon a circular dial-plato, is applicable to spring-balances of other than the helical construction. It was used by M. Manila, a French gentleman, who was rewarded by the Society of Arta, in 1790, for an apparatus for showing at one view the weight of an object according to several different scales or systems of weights. His machine which is described and figured In the ninth
volume of the Society's ' Transactions,' consists of a dial-plate, on which are marked several concentric circles, divided according to the systems of weights used in different countries, and an index moved by a rack and pinion, as before described. The spring, instead of being of a helical form, is semicircular ; its upper extremity being firmly attached to the back of the dial-plate by means of screws, while its lower end is attached to the hook which carries the weight, and the eliding-rack by which the index is moved. Marriett's patent weighing machine is similar to that of M. Hanin, but the spring is a perfect ellipsis, with its longer axis laid The stem to which the ring for bolding the apparatus is attached is fastened by a out and screw to the middle of the upper aide of the spring; and the rack, with the hook which holds the article to be weighed, to the correspond ing point on the lower side of the spring. The spring, rack, and pinion are enclosed in a circular box at the back of the dial-plate, the periphery of which serves as a stop to prevent the spring from being overstrained. A similar apparatus, contrived by M. Regruer, has been used as a dynamometer, as well as a weighing-machine.
A scale-plato or dish may be added when necessary to any of the spring weighing-machines which have been described. On account of the absence of weights, and the great simplicity of their application, spring-balances are useful in cases where extreme accuracy is not required, especially when a portable weighing-machine is desirable. Machines for ascertaining the weight of the human body are often made on this principle, a kind of chair being suspended from the spring.