Excellent! I see you've grasped the point, Gene. Mercury thermometers do indeed measure changes in volume of the contained mercury. If that mercury were replaced by another fluid, the marks on the barrel would tell a lie. So, mercury thermometers are another example of devices used for "indirect measurement". Other examples are of course alcohol thermometers, electronic thermometers, and infrared intensity measurements taken by our weather satellites. For those devices and others (such as thermocouples, RTDs, etc.) the direct measurement may be a volume change, an electric potential, electrical resistance, luminous intensity, and so forth. We must then infer temperatures from those measured quantities. Calibration is often two-fold. The first, usually done at the point of manufacture, is to calibrate the device for the quantity directly measured. The second, often checked in the field, is the calibration of the scaling factor used to relate the measured quantity to temperature. Temperature measurements are usually inferred, indirect measurements. So is mass. Thanks for providing that supporting example, Gene; I think it will help people see the point. Jim Gene Mechtly wrote: > > On Sun, 8 Jul 2001, James R. Frysinger wrote: > > > ... most commercial and laboratory devices actually measure force, ... > > Not true. Devices measure the quantity they are calibrated to measure. > > Load cells are compressed in response to forces, but thousands of them > are legally *calibrated* to measure mass in market applications. > > Similarly, a mercury thermometer is *calibrated* to measure temperature, > but the actual process is change in volume of the mercury. > > I doubt that you would argue that a thermometer "measures" volume changes. > > 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
