Editing Weighing scale (section)

====Spring scales====
{{main|Spring scale}}
A [[spring scale]] measures mass by reporting the distance that a [[spring (device)|spring]] deflects under a load. This contrasts to a ''balance'', which compares the [[torque]] on the arm due to a sample weight to the [[torque]] on the arm due to a standard reference mass using a horizontal [[lever]]. Spring scales measure [[force]], which is the [[tension (physics)|tension]] [[force]] of constraint acting on an object, opposing the local [[gravity|force of gravity.]]<ref>{{cite web|url=https://www.inscale-scales.co.uk/blog/guide-to-choosing-best-mechanical-scale/|title=A Guide to Choosing the Best Mechanical Scale – Inscale|website=Inscale Scales|language=en|archive-url=https://web.archive.org/web/20171206203111/https://www.inscale-scales.co.uk/blog/guide-to-choosing-best-mechanical-scale/|archive-date=2017-12-06|url-status=dead|access-date=2017-12-06}}</ref> They are usually [[Calibration|calibrated]] so that measured force translates to mass at earth's gravity. The object to be weighed can be simply hung from the spring or set on a pivot and bearing platform.

In a spring scale, the spring either stretches (as in a hanging scale in the produce department of a [[grocery store]]) or compresses (as in a simple bathroom scale). By [[Hooke's law]], every spring has a proportionality constant that relates how hard it is pulled to how far it stretches. Weighing scales use a [[spring (device)|spring]] with a known spring constant (see [[Hooke's law]]) and measure the displacement of the spring by any variety of mechanisms to produce an estimate of the [[gravity|gravitational]] force applied by the object.<ref>{{Cite news|url=https://phys.org/news/2015-02-law.html|title=What is Hooke's Law?|access-date=2017-12-06}}</ref> Rack and [[pinion]] mechanisms are often used to convert the linear spring motion to a dial reading.

Spring scales have two sources of error that balances do not: the measured mass varies with the strength of the local gravitational force (by as much as 0.5% at different locations on Earth), and the elasticity of the measurement spring can vary slightly with temperature. With proper manufacturing and setup, however, spring scales can be rated as legal for commerce.  To remove the temperature error, a commerce-legal spring scale must either have temperature-compensated springs or be used at a fairly constant temperature. To eliminate the effect of gravity variations, a commerce-legal spring scale must be calibrated where it is used.