DYNAMOMETER, from Juvum.G strength, and pArreov a measure, is the name of an instrument for measuring the relative strength of men and animals.
The dynamometer invented by Mr George Grahame, and improved by Dr Desaguliers, consists of a strong frame of wood, ABCD, (Plate CCXLII. Fig. 1.) Through the piece DC is a hole 1), sufficiently large to admit a cylin drical iron bar, about an inch in diameter. Upon this bar is a square to receive the two separate and unequal arms of a bent lever DI', DE, which are kept tight in their place by a strong screw nut d. The arm DE, which carries a weight \V, is prevented from falling be low a horizontal position, by a metallic pin at K, which stops the arm DF in its progress towards C, but both the arms move freely in the opposite direction. At the top of the arm DF, seen separately at df, is a round cross bar about six inches long. The iron piece LN, also seen separately, has likewise a cross bar at top, and holes for iron pins to fasten it in its place. Another piece of iron seen separately at h gi, is fastened to the tim ber that carries the lever by a strong wooden screw at I, and by the pin K going through its wings and the tim ber. The collar S is to be put on when the upright arm of the lever is not used, and Al is the centre of gravity of the steel-yard DE.
In using this machine, the person who wishes to try his strength, must take hold with his left hand of the round part of the cross at N, and of the round part of the cross at F with his right hand, and then by bring ing his right hand towards the left, in the direction FN, he will move DE, and elevate the weight W. \Vnen this weight is lifted up so as to make the arm FD just quit the pin at K, the force of the arm will be determined in the following manner: Suppose the weight NV to be 56 pounds, and the distance from the fulcrum, viz. WD, fifteen inches, then the momentum of \V will be 56x15=840. Let us suppose also, that it requires six pounds applied at M, the centre of gravity of the steelyard, to balance the steelyard itself. Then if I\ID=10, we shall have 6x10=60 for the additional resistance made to the force of the arm ; so that the whole resistance will be which divided by 1:13=1C inches, the distance of the power, will give ninety pounds for the force of the man's arms when ap plied at 1' and N. If another man is capable of raising double the weight \V, added to double the weight of the steelyard at AI, he will be twice as strong. In
stead of increasing the weight at \V, the weight may be removed towards E. Desaguliers has described se veral variations in the construction of this machine, and has also given a drawing and description of an in strument for measuring the strength of the fingers; but for an account of these, we must refer the reader to his Course of Experimental Philosophy, vol. i. p. 291, 292. Annot. on Sect. iv.
The dynamometer invented by Leroy, of the Acade my of Sciences, consisted of a metal tube, ten or twelve inches long, placed vertically on a stand, and contain ing a spiral spring, having above it a graduated shank terminating in a globe. This shank, together with the spring, was pressed into the tube in proportion to the force which was applied to it, and pointed out upon the graduated shank the strength of the person who exerted the force.
The most valuable dynamometer, however, is that which was invented by Regnier, and of which we have given a representation in Plate CCXLII.
This instrument, which resembles a common grapho meter in its form and size, consists of a spring AA (Fig. 2.) twelve inches long, and bent into the form of an ellip sis. This spring is covered with leather to prevent it from hutting the fingers when strongly pressed with the hands. It is composed of the best steel, excellently weld ed and tempered, and subjected to a greater force than it ever can be exposed to when in use, in order that it may not lose any of its elasticity by frequent use. A piece of steel B, Fig. 3. is firmly fastened to the spring by means of a claw and screws, and is intended to sup port a semicircular plate of brass C, Fig. 2. for receiving the scales or graduated arcs. The outermost of these arcs is divided into myriogrammes, and the other into kilogrammes. Each of these arcs is still farther subdi vided by points, which express the weight in pounds mare; and the various parts of the scale are determined experimentally, by appending accurate weights to one of the extremities of the elliptical spring. On the other branch of the spring, is a small steel support D, furnish ed] with a horizontal cleft at its upper extremity, to re ceive freely a small copper lever E, which is kept in its place by a steel pin a. This part of the machine is re presented on an enlarged scale in Fig. 4.