106 6
THEORY AND PRACTICE OP ENGINEERING.
Rook II.
The dvnanometer just described indicates on the scale of traction a tension of a tonweight, which is greater than the most powerful effort of a horse, but nevertheless toosmall to measure the ordinary effort of a power applied to a screw.
M. Regnier constructed a dynanometer on the same principles, which measures a tractionof 6600 lbs. : the spring was of the same power and length as the old one, but the twoarcs or plates of which it was composed were longer, thicker, and further apart in theircentres ; their distance from each other was 4-5 inches; by the greatest traction it was onlydiminished -4 of an inch ; these arcs had in the middle a breadth of I '8 inches and a thick-ness of *2 of an inch ; the total weight of the instrument was 5\ lbs.
By placing the machine between the two ends of a cord passing over two or morepulleys, the ratio of the force which separates the extreme pulleys to the tension of the cordwill be known, and by this disposition we can measure a traction much more considerablethan that which is indicated by the scale of the dynanometer. Pig. 1633. shows the arrange-ment with two pulleys.
Details of the Construction of the Dynanometer. —Two supports D, D', of steel, areadjusted solidly on the two opposite branches of the spring in the direction of the per-pendicular of the axis. The first support D, cut in a fork, carries a screw on whichthe extremity a of the counterpoise ah rolls; it is about 1 ’4 inches high, and *59 wide; it isretained on the centre of the arc A B by a strong screw, whose head is marked T on theconvex part of the arc. The second support D is also retained by a screw, the extremityof which, t, is seen oh the concave part of the arc A’ B’; the upper face, E, E, of this supportis about 4 inches long: on the opposite face, E P, which is of the same length, is abrass plate, I L, fixed by a single screw g, which is level with E F in g: the plate, hardenedto make a spring, carries at its extremity a pivot I, which passes through the support andthe limb; this pivot, like that of a compass-needle, serves as a centre to the bent lever b H c ;the limb is applied to the face E F, of the support 1)', and is fixed there by two screws e,fThe plate IL being a spring, the pivot I yields to a pressure of the counterpoise, and pre-vents any rupture of the mechanism which turns the needles of the scales.
The covering plate OPQ is voided at K' in a small circle of a diameter nearly equal tothat of the head of the screw K, round which the needle Krfd works, with a slight frictionon the limb : if this friction be too slight, a turn-screw which passes through the circularopening K'will tighten the pressure. The lower pivot of the bent lever 6Hc rolls on thepin P; its upper pivot rolls on the side H', which is riveted to the covering plate OPQ.
Spring Balance and Eprouvette. —The spring balance most used in commerce is formedof two steel branches AC, C B, bent at an angle of 45° ; each of the arcs Dp q E, 111 r s G,is fixed to one of the branches and traverses the other.
By drawing the rings E, G, which terminate the ares inopposite directions, we bring the branch A C near B C; acircular scale figured from 5 to 40 indicates the respectivepositions of these two branches. The branch A C pushesbefore it a small cursor k of card or leather, which slideseasily on the metallic wire/p, attached to the branch CBof the balance. To graduate the scale, suspend the balanceby a ring E fixed to the branch A C, and attach weightsto the ring G, which is at the extremity of the scale. Thenumbers on the scale indicate the tension of the spring.
Regnier has made an excellent instrument of this spring-balance for trying the strength of powder. The lengthof the branches A C and C B is about 4*8 inches, and theirbreadth about an inch ; a small brass cannon, whose breechII is on the branch C B of the balance, and whose mouth Iis closed by the fuse I E of the obturation DI L E fixed onthe other branch A C of thebalance, contains a given weightof powder to be tried ; it is primed by a little powder put in the pan F : the powderwithin the cannon is fired and drives it away; after the ignition the two branches of thebalance approach, and the cursor k indicates on the scale the tension of the spring at themoment of the explosion. The iron D E, and the brass arc GII on which the scale isdrawn, pass through openings pq, rs made in the middle of the plates C B and C. A.
A dynanometer for measuring animal force was invented by Mr. Graham, and afterwardsimproved by Desaguliers , but it was found to be inconvenient for the purpose, as it wasmade of wood; Leroy afterwards formed one in a metal tube 10 or 12 inches in length,with a spring within it placed vertically on a stand; this, however, was not found so usefulas Regnier’s, where the sides of the spring are made to approach each other, and move anindex, which marks the degree of approximation on a semicircular scale ; a man can ascer-tain by it the mean force he exerts with either hand, or -with both together ; it was with thiskind of dynanometer that M. Quetelet, of Brussels , made his experiments upon the strengthof men of different statures and ages.
Fig. 1634.