The Alden friction brake and the Froude hydraulic brake suggest another form of dyna mometer especially useful for absorbing the power developed by high speed machinery such as steam turbines. This machine, which was designed by Prof. J. B. Webb, consists of a series of steel discs, such as old circular saws, connected to a rotating shaft having separating washers between the discs. The shaft and discs rotate within a fixed casing to which a series of annular discs is secured in the rim; these are so spaced that they alternate with the rotating discs on the shaft but are not in contact with them. The casing is filled with water which acts as a drag and tends to rotate the stationary discs and hence the casing. The tendency to rotation can readily be determined by placing weights upon an arm attached to the casing from which the power of the tur bine or other motor may be found by the usual methods. The temperature of the water may be regulated as desired by controlling the flow through the dynamometer.
All of the preceding dynamometers absorb the power delivered to them; in order to trans mit the power from a prime mover or other source, to a machine or group of machines, some form of transmission dynamometer must be used.
Morin, one of the early users of dyna mometric apparatus, gave as the requirements of a dynamometer the following: °First. The sensibility of the instrument should be proportioned to the intensity of efforts to be measured, and should not be liable to alter ations by use. Second. The indications of flexures should be obtained by methods inde pendent of the attendance, fancies or prepos sessions of the observer, and should conse quently be furnished by the instrument itself, by means of tracings, or material results, re maining after the experiments. Third. We should be able to ascertain the effort exerted at each point of the path described by the point of application of the effort, or, in certain cases, at each instant in the period of observations. Fourth. If the experiment from its nature must be continued a long time, the apparatus should be such as can easily determine the total quantity of work expended by the motor.' To meet these conditions Morin made the spring-dynamometer, in order to obtain the magnitude of a force, as for instance, the trac tion of a horse on a loaded wagon or canal boat.
In this dynamometer a force was measured by the flexure produced by it on two springs connected at their ends and loaded in the mid dle. The force is applied at the centre of spring, and its magnitude is determined by the increase of the distance between the two springs when one of them is attached to its load at the centre. In order to meet the second, third and fourth Morin designed a self registering recording dynamometer, by which the work performed was traced upon a continuous roll of paper set in motion by suitable wheel work. Such dynamometers are in:use at the present time to determine the tractive .efforts of animals or gas tractors in the field or with loaded wagons on roads of different character.
When required to determine the force of rotation of a shaft or nulley this form of dyna mometer requires modification; the essential features, however, remain the same. In this case a pulley, free to rotate on a shaft, is con nected to it by means of two parabolic springs which are fastened to the shaft and also to the rim of pulley. These springs turning with the shaft deflect more or less, according to the re sistance encountered, and when the resistance to bending overcomes the resistance of the machine, motion is transmitted through the springs to the pulley and thence to the machine under load.
Using the same notation previously given, and substituting R — radius in path in feet— for, L the work done is W = 2 r RNP; where P = resistance overcome in the machine driven by the dynamometer. P can be readily ascer tained when deflection of spring is known.
Another form of transmission dynamometer, sometimes called the differential Onamometer, has long been in use. In this machine it is as sumed that it requires as much power to hold a weighted radius arm horizontal as it does to lift the same weight through the distance which would be traversed by it in any given number of revolutions if rotated in the circle and in the time required for such number of revolu tions. This it will be seen is similar in prin ciple to the method used in determining power by means of any friction brake, or absorption dynamometer; but while the latter absorbs the power, the differential dynamometer measures or weighs the power transmitted through it.
Still another form of transmission dynamom eter by which the to rotation is weighed is that designed Prof. S. W. Robinson. This machine consists essentially of a supporting frame, or pedestal, a T-shaped arm carrying the driving mechanism, and a graduated scale or weighing apparatus. In use, the lower pulley of the dynamometer is belted to the machine to be tested, while a second belt connects the upper pulley with the one on the power shaft. The two pulleys of the dyna mometer are mounted on a strong cross-tree bar so that they both overhang and can be swung around to any position. These pulleys have each a gear on the end of the hub, both 'of which mesh into a smaller gear between, the latter being supported on a pin made fast as a crank-pin in an arm attached to a shaft which passes through the centre of the hub of the cross-tree. To the opposite end of this shaft the poise-bar is made fast by a set screw in a boss to which the poise-bar is secured. As work is transmitted from one pulley to the other through the intermediate gear on the crank-pin, the latter will be thrust to one side with a force proportional to the effort trans mitted; this tends to tip the poise-bar which is prevented by adding weights. The equilibrat ing weights on the poise-bar, together with the speed, furnish data for calculation of the work being transmitted.