A balance scale compares masses, it does not measure force due to gravity. You put the object to be weighed on one end of the balance. You then add weights of known mass on the other end of the balance, until the balance is level. This procedure requires acceleration due to gravity to work, but doesn't depend on the actual value of the acceleration. So it is a measure of mass, not force due to gravity. -- SJK
Disagree. You can use the exact same device to measure charges in an electromagnetic field, or the relative strengths of two springs. It's only measuring mass when you decide to interpret the results as mass; when you decide to interpret them as forces, it's measuring forces. It doesn't make any sense to talk about what a device is "truly" measuring when it's measuring two coincident things! I really think this article's emphasis on whether or not usages are correct, rather than what weight is, is a bad thing, and would vote for this passage to be removed and others to be rewritten, but I don't want to get into a back and forth edit war.
The comments still apply. Ancient peoples couldn't be talking about mass as opposed to gravitational force if they didn't have distinct concepts of each. Scales measure mass when you use masses for reference and forces when you use forces for references, and so to speak of them truly measuring one or the other is silly, modulo KA's comments above. And, btw, does anyone know if there is an official usage for the pound in Britain or elsewhere? America hardly determines universal standards for units, as the gallon proves.
Ok. Well, one still finds a considerable body of literature using pounds exclusively as a unit of force, especially in derivative units (e.g. 550 foot-pounds/second = 1 horsepower), and it would be somewhat hypocritical to talk about historical usage and then turn around and label these as simple obsoletes. I think that, when usage is varied, an article should reflect varied usage and not try and impose a false order on reality.
An example of a case where common usage refers to force, and not mass, is when people say objects in space are weightless.
Also, yes people in past centuries weren't entirely clear on the difference between mass and force due to gravity, but most of the time it was mass, not force due to gravity they were worrying about. If you weigh out 5 troy ounces of gold, do you want 5 ounces mass or 5 ounces force due to gravity? You are interested in the mass, not the force due to gravity. Similarly, if an apothecary weighed out 1 grain of some medicinal substance, they wanted 1 grain mass, not 1 grain force. Weighing is (and more importantly was) most commonly done to determine the amount of substance, not to determine how heavy it is. So even if they weren't entirely clear on the difference, they wanted mass. -- SJK