Good answer, Bill, but one can take this farther. Some balances operate by deflecting a countermass away from equilibrium and the deflection operates linkages and gears which communicate mechanically to the dial needle. I have a wall balance that uses this principle. Such balances are unaffected by the magnitude of the local gravitational field, if properly designed, since both the test object and the countermass experience the same field. Thus they would work on the moon, for example, as would a triple beam balance. To be very technical, however, even those measure a force. If no gravitational field at all is present, they will not operate properly. The test object and the countermass must experience gravitational forces of attraction (i.e., weights) else there would be no forces to balance. Newton's laws speak of balanced forces (as used by balances) but there is no law regarding a balance of masses. Thus, these devices actually measure force (weight), as do devices which use strain sensors or springs. In the former case, it is the mechanical advantage of the linkage system which is being tested during calibration and in the latter case it is the characteristic property of the strain sensor or spring. To truly measure mass, one might use an inertial oscillator, which actually measures something depending on the inertial properties (mass) of an object. Some time ago, I mentioned a technique to build one out of hacksaw blades, C-clamps, and a pan. Such a device would operate in freefall (such as in the International Space Station) or in the emptiest regions of space. Many science books show the technique. Even these devices require calibration due to their dependence on the properties of the deflected blades. So, most commercial and laboratory devices actually measure force, since they require a gravitational field to operate. HOWEVER, this is of no great concern to most people. As we've said here many times, weight is often taken to be synonymous with mass for most commercial purposes (the framework of Joe's comments) and this is well supported in various standards. Usually it is only in the case of technical work that one needs to distinguish the two quantities--or for the sake of pedantic rectitude and pontification. Jim Bill Potts wrote: > > Gene: > > The only scales that measure mass are balance scales, because they balance > something with something else whose mass is known. They will produce the > same result on Earth, on the Moon or on Mars. As Joe has implied, Toledo > scales use a balancing mechanism. (I actually have some doubts about Toledo > scales, though; with true balance scales one takes note of the reference > mass at the point of zero deflection, whereas the Toledo scales measure by > deflection.) > > Commercial scales using springs or transducers measure the downward force, > due to gravity, of the mass placed on them. They will produce different > results on the Earth, on the Moon and on Mars. They measure weight. > > However, given that they are calibrated on the Earth and in the general > geographic area where they are to be used (i.e., given the constancy of the > ratio of measured force to actual mass), the units displayed can reliably > indicate the mass placed on them. > > Bill Potts, CMS > Roseville, CA > http://metric1.org [SI Navigator] > > > -----Original Message----- > > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On > > Behalf Of Gene Mechtly > > Sent: July 08, 2001 12:47 > > To: U.S. Metric Association > > Cc: U.S. Metric Association > > Subject: [USMA:14208] Re: Constitutionality > > > > > > On Sun, 8 Jul 2001, Joseph B. Reid wrote: > > > ... > > > I meant scales that weigh in both pounds and kilograms should not be > > > certified. Those scales are electronic and switchable between > > pounds and > > > kilograms. Those scales measure weight. Mechanical scales like Toledo > > > ("Honest weight, no springs") measure mass. > > > > Nonsense, Joe. All honest scales (calibrated for legal trade), > > both mechanical and electronic, measure mass and have measured mass for > > many decades whether consumers have been aware or not that both pounds > > and kilograms are legally defined as units of mass. > > > > Gene. > > -- Metric Methods(SM) "Don't be late to metricate!" James R. Frysinger, CAMS http://www.metricmethods.com/ 10 Captiva Row e-mail: [EMAIL PROTECTED] Charleston, SC 29407 phone/FAX: 843.225.6789
