MACHINES FOR THE MEASUREMENT OF MOTION.
Measurement of for measuring power of any kind —for example, the strength of men and of draught animals, the force exerted through machinery, etc.—are called "dynamometers," which may be divided into two general classes: (1) those for the measurement of power exerted by a prime mover, and (2) those for the measurement of power transmitted. In the first class are those contrivances in which the power is exerted on a lever or an elliptical spring, and in the second class are those mechanical devices in which the force transmitted from a prime mover or the amount of force consumed in driving a machine or a series of machines is determined. They involve, generally, the expedient of interposing between the motor and the machine, as a medium through which the power is to be transmitted, sonic combination of springs and a scale on which arc recorded the degrees of static force corresponding to different states of tension, and sometimes also an automatic mechanism for making periodical record of the marking of the index on the scale, or some mechanism whose essential parts are a lever and a weight. The require ment of a perfect dynamometer is that it shall not itself be a charge on the power—that is, that by its interposition the expenditure of driving force required shall not be sensibly increased. This property belongs to all that class in which the power of the motor acts directly with all its force to produce flexure in springs, while the springs, by their effort to recoil, transmit the force undiminished to the machine.
Prony form of dynamometer for measuring the effect (horse-power) that a motive machine imparts to the transmitting shaft, called, after its inventor, the Prony dynamometer or friction brake, is shown in Figure 5 (ftV. 127). It consists of two friction blocks (E, F) and a lever (C) carrying on its outer end a scale (D). The friction blocks are screwed to the journal to be tested, and tightened by the screws G until the unweighted lever stands in a horizontal position. The shaft is now set in motion by the prime mover, the screws G are gradually tightened, and weights are placed at D until, with a horizontal lever, the shaft revolves with the desired velocity. From the amount of weights placed on the scale D and the number of revolutions indicated, the effect imparted to the shaft by the motor can easily be calculated. To prevent the lever from being whirled around in a circle by the friction of the shaft if the scale. be not sufficiently weighted, the stationary studs!, I are provided; moreover, soapy water is poured through the funnel H on the circumference of the journal to, prevent its running hot, and also to prevent the possible igni tion of the braking-blocks by the frictional resistance.
The Balance Dynamometer (jigs. 6, 7), an invention of Samuel Batchelder of Boston, is placed in the line of communication between the motor and the machinery to be moved; the power exerted on the machinery may be exactly measured by means of the steelyard and weight. There is also connected with it an index to show the number
of revolutions of the drum for a given time, which, being observed together with the weight, gives the data for computing the number of pounds of static force exerted at the time on the dynamometer. The machine receives its power from the prime mover by a belt on the pulley A, and the power is transmitted to the machine (which is the subject of experiment) by a belt from the pulley B. The first pulley A and the bevel wheel D are fast on the shaft C, which revolves in bearings I. The bevel wheel F is connected with the pulley B by a sleeve A', which is capable of turning on the shaft C. The bevel-wheels DF are geared together by the bevel-wheels EE, which run on a cross shaft having a boss G, through which the main shaft passes freely. It is evident that if this cross shaft is not retained in its place by some adequate force, the motion of the bevel-wheel D will only cause the cross shaft to move round on the shaft C, and the wheels E will roll on the wheel F without communicating motion to it or to the pulley B; but if the wheels E and the cross shaft are held stationary, the motion of the pulley A will be communicated to the pulley B through the bevel-wheels, and the force there applied to retain the shaft G and the wheels E in place will indicate the power transmitted through the dynamometer. The amount of power is ascer tained by means of a graduated scale-beam H,J, connected with the shaft of the wheels E by straps a. The weight Jit; fastened to the shorter arm of the graduated beam by a set-screw, affords a means of balancing the beam when the machine is at rest, and the weight I-V, like that of a com mon balance, when moved on the graduated arm of the lever, will indi cate the strain on the belt. The number of pounds thus indicated, multi plied by the number of feet through which the belt moves per minute, will give the number of pounds raised one foot per minute; the prod uct, divided by 33,000, gives the horse-power expended in driving the machinery. (Contrivances for indicating the action of steam arc consid ered in the sections on steam-boilers and steam-engines.) Measurement qj Speed: determining the velocity of the wind there arc employed various devices called "anemometers." They consist for the most part of a " fly " or small wind-wheel and shaft that actuate a train of gearing which causes a pencil to trace a record of the velocity, as also the direction, of the wind on paper specially ruled for the purpose. In one form of the machine the recording paper is placed on a stationary cylinder about which the pencil is moved as the wind changes its direction; in a modified form the paper is slowly moved by clock-work.