Mechanical balances Weighing scale

two 10-decagram masses



market hawker weighing meat (in kati) on beam balance.



masses of 50, 20, 1, 2, 5 , 10 grams

unlike spring-based scales, balances used precision measurement of mass accuracy not affected variations in local gravitational field. (on earth, example, these can amount ±0.5% between locations.) change in strength of gravitational field caused moving balance not change measured mass, because moments of force on either side of beam affected equally. balance render accurate measurement of mass @ location experiencing constant gravity or acceleration.

very precise measurements achieved ensuring balance s fulcrum friction-free (a knife edge traditional solution), attaching pointer beam amplifies deviation balance position; , using lever principle, allows fractional masses applied movement of small mass along measuring arm of beam, described above. greatest accuracy, there needs allowance buoyancy in air, effect depends on densities of masses involved.

aluminum, mass-produced balance scale (steelyard balance) sold , used throughout china: scale can inverted , held larger ring beneath user s right hand produce greater leverage heavier loads (hainan province, china)

to reduce need large reference masses, off-center beam can used. balance off-center beam can accurate scale center beam, off-center beam requires special reference masses , cannot intrinsically checked accuracy swapping contents of pans center-beam balance can. reduce need small graduated reference masses, sliding weight called poise can installed can positioned along calibrated scale. poise adds further intricacies calibration procedure, since exact mass of poise must adjusted exact lever ratio of beam.

for greater convenience in placing large , awkward loads, platform can floated on cantilever beam system brings proportional force noseiron bearing; pulls on stilyard rod transmit reduced force conveniently sized beam.

one still sees design in portable beam balances of 500 kg capacity commonly used in harsh environments without electricity, in lighter duty mechanical bathroom scale (which uses spring scale, internally). additional pivots , bearings reduce accuracy , complicate calibration; float system must corrected corner errors before span corrected adjusting balance beam , poise.

a roberval balance. pivots of parallelogram understructure makes insensitive load positioning away center, improves accuracy, , ease of use.


roberval balance

in 1669 frenchman gilles personne de roberval presented new kind of balance scale french academy of sciences. scale consisted of pair of vertical columns separated pair of equal-length arms , pivoting in center of each arm central vertical column, creating parallelogram. side of each vertical column peg extended. amazement of observers, no matter roberval hung 2 equal weight along peg, scale still balanced. in sense, scale revolutionary: evolved more-commonly encountered form consisting of 2 pans placed on vertical column located above fulcrum , parallelogram below them. advantage of roberval design no matter equal weights placed in pans, scale still balance.

a gear balance: = axle, f = frame, g = generator, gl = geared linkage, wl = weighted lever; counter weight added balance, gear linkages free running on rotating frame

further developments have included gear balance in parallelogram replaced odd number of interlocking gears greater one, alternating gears of same size , central gear fixed stand , outside gears fixed pans, sprocket gear balance consisting of bicycle-type chain looped around odd number of sprockets central 1 fixed , outermost 2 free pivot , attached pan. because has more moving joints add friction, roberval balance consistently less accurate traditional beam balance, many purposes compensated usability.