709 



DYNAMOMETER. 



DYNAMOMETER. 



710 



MOTION, LAWS OF, &c. &c., the details of each subdivision will be 

 clearly understood from the mathematical proofs and explanations 

 there given. The history of dynamics is particularly connected with 

 the names of Galileo, Huyghens, Newton, D'Alembert, and Lagrange, 

 whose lives are given in the BIOG. Drv. See also on this point 

 MECHANICS, the general term under which statics and dynamics are 

 included. 



DYNAM'OMETER (Svva/u! and nitfov, measurer of power), a term 

 which has been applied to an instrument which measures any thing to 

 which the name of power has been given, whether that of an animal, 

 or (to take a very different instance) of a telescope, only in the latter 

 case it is termed a dynameter. [MICROMETER.] We have also seen the 

 incorrect term dynometer. 



Dynamometrical instruments have been but little attended to in 

 this country, but they have been used successfully in France for deter- 

 mining the best conditions, with respect to economy of power, under 

 which machines may be constructed and worked. The sagacity of 

 Watt led him to appreciate the importance of a dynamometrical 

 machine, and he constructed his indicator for determining the work 

 performed by the steam on the piston of a steam-engine during any 

 single stroke. Prony's Friction-Break measured the work by which a 

 shaft was driven when all other work was thrown off it. In the 

 later constructions, however, the object has been to determine the work 

 transmitted to the whole or any part of a machine, not for a single 

 stroke of the piston, but continuously through any given period, and 

 that without absorbing the whole work to be measured as the friction- 

 break does, but while the machine is performing its usual work. 

 Dynamometers of this kind have been contrived almost entirely by 

 French engineers, and an account of them will be found in the 5th 

 edition of CoL Morin's ' Aide-Me'moire de Mccanique Pratique,' and 

 we may also refer to the ' Lecons de Mdcanique Pratique," of the same 

 author. A good concise statement of the principles of these instru- 

 ments is given in Nichol'a ' Cyclopaedia of the Physical Sciences," 1857, 

 and a detailed description of the dynamometer as originally invented 

 by General Poncelet, and constructed under the direction of Colonel 

 Morin, is given in Tomlinson's ' Cyclopaedia of Useful Arts," Introduc- 

 tion, p. clii. 



In the Great Exhibition of 1851, the Jury of Class V. (Machines for 

 Direct Use,) were assisted by Morin's Dynamometer in examining and 

 reporting upon a number of machines exhibited, in which the same 

 object was sought to be attained by various modifications and com- 

 binations of similar parts. There were several kinds of dynamometers 

 exhibited, such as the Di/namomttre de Rotation for determining the 

 work transmitted by a revolving shaft. There was also a dynamometer 

 for determining the manual labour of driving the handle of a pump or 

 crank : also a dynamometer for registering the work of the steam on 

 the piston of an engine, through any number of strokes ; this was con- 

 structed on the principle of the constant indicator [INDICATOR] made 

 for the British Association of Science, and described in their Report 

 for 1841. 



The principle upon which an efficient dynamometer depends may 

 be briefly stated : We may regard farce as something instantaneous ; or 

 changing, or beginning, or ceasing at any moment!; and work, as the opera- 

 tion of forces through spaces of time. Thus, if a body be pulled along 

 a level road, force is exerted every moment, and if we pull another 

 body with half the force over twice the distance, the same amount of 

 work will have been performed. Work then depends on the force 

 applied, and the space through which a body subjected to the force 

 moves, and an efficient dynamometer measures the work in any case of 

 resistance overcome and motion produced. The work will be measured 

 by the product of the force (if kept uniformly acting) into the space 

 through which it acts, and the instrument must measure both these 

 quantities the force and the space. 



The original dynamometer simply determined the value of the force 

 as in the instrument invented by Grahame and improved by Desa- 

 guliers, for ascertaining the relative strength of an individual at 

 different periods of life and in different states of health. It consisted 

 of a steel yard mounted in a wooden frame, and the strength of an 

 individual was measured by acting on the hort end of the lever, so as 

 to produce equilibrium, and the position of the weight on the long 

 arm served as a measure. An instrument by Leroy consisted of a 

 metal tube 10 or 12 inches in length placed vertically on a foot, like 

 that of a candlestick, and containing a spiral spring, and above it a 

 graduated shank terminating in a globe. The shank, together with 

 the spring, retreated in the tube in proportion to the weight acting 

 upon it, and thus pointed out in degrees the strength of the person 

 who pressed on the ball with his hand. Kegnier's dynamometer consists 

 of a steel spring bent into the form of an ellipse, and furnished with a 

 semicircular plate of brass for receiving the scales or graduated arcs ; 

 these are fastened to one limb of the spring by means of a piece of 

 steel, while to the other limb is attached a small steel support with a 

 cleft at its upper extremity for receiving a small copper lever, which 

 gives motion to the index needle on the scale. The scale is subdivided 

 by hinging known weights to one of the extremities of the spring. 

 Any force acting in the direction of the longer axis of the ellipse 

 tends to flatten it, and giving motion to the lever, the needle moves 

 over the scale, and shows the amount of force exerted. In trying the 

 strength of the human body, the feet are placed in a kind of rack, to 



which one end of the dynamometer is attached, while the other end is 

 furnished with a double handle, and the person under trial exerts the 

 strength of the reins of his body in compressing the ellipse, the 

 amount of force exerted jbeing shown by the index. In determining 

 the strength of the hands or the muscular force of the arms, the 

 person holds the two sides of the spring nearest to the centre, and 

 exerts his force in flattening the ellipse. In ascertaining the strength 

 of animals the instrument is attached to a fixed obstacle, and the 

 animal is yoked to the other extremity. [ANIMAL STRENGTH.] There 

 are various other forms of dynamometer of this kind for determining 

 simply the value of the force, without reference to the space through 

 which it acts, and this would be sufficient if the force were constant, 

 for all that would be necessary would be to measure the whole space 

 and the one force ; but as working forces are by no means constant, 

 but are subject to frequent variations during the times when it is 

 required to measure the work, we must take the small spaces in 

 which the forces are tolerably constant, and multiply them by the 

 forces which act as they move through these spaces, and by summing 

 up the amounts of work contained in each, arrive at the total value. 



We shall be able to understand how both results are obtained by the 

 following detailed description of Morin's dynamometer. When applied 

 to measure the mechanical force transmitted by a wheel or pulley 

 intervening between a source of power and the machine which is to be 

 tested, the form of the instrument is that represented in Jig. 1 (figs. 2 

 and 3 being parts of the same instrument in different points of view), 



Fig. 1. 



in which A is a stout axle of iron, turning freely in its bearings, and 

 carrying a pulley or drum c. This pulley is keyed to the axle, and 

 turns with it, while a second pulley D, is placed on the same axle, but 

 is not fixed or connected therewith, except by means of the spring E. 

 One end of this spring is attached to the axle, while the other is held 

 firmly by two cheeks projecting from the side of the loose pulley D. 

 Now it will be evident that if any resistance act on the pulley c, to 

 prevent it from rotating, and a force be applied to D to overcome such 

 resistance, the force could only act on c, by means of the spring E, 

 which would be bent until its resilient tendency became just equal to 

 the resistance acting upon c. The pulley D not being connected with 

 the axle, except through the spring E, transmits force only by the 

 flexure of the spring, so that if the resistance on c were continuous, 

 such as would arise from c being made to drive a machine, and the 



