For that matter, force cannot be directly measured. Nor can time.
Electric current could be measured directly if one could measure time
and could count individual elementary charges passing a gate. The
latter two quantities are also part of the "magnificent seven" whose
units provide the base set in SI.
I could go on but I'll be merciful and resist the urge to list more
quantities which cannot be directly measured. But the bottom line is
that we, as people, "invent" these quantities to describe nature and
then "invent" tests that we use to measure them. Call it indirect
measurement if you wish.
Jim
On Tue, 27 Feb 2001, John Tauxe wrote:
> You all are missing my point.
> Let's see, how can I make myself more clear...?
>
> Mass itself cannot be measured directly, it can only be inferred by the
> behavior of a mass undergoing acceleration. None of the schemes proposed
> here (the balance, the calibrated oscillation -- though that is clever and
> would work in zero gravity) measure the mass directly.
>
> The balance compares the weights of two (or more) objects. If they exhibit
> the same weight, then they must have the same mass. Still the mass itself
> has not been measured, it has only been compared to a standard. (And
> whether that standard was water or an alloy is irrelevant to this
> discussion.)
>
> The oscillator (essentially a mass on a spring) also does not measure mass
> directly. What is measured is the period of the oscillation. Again, this
> can be compared to a standard, but the mass is not being measured.
>
> I think it's kind of neat that mass cannot be measured. No one need be
> defensive about this!
>
> - John
>
> P.S. I'll have to retract my statement "Show me a device that can measure
> mass under zero gravity and I'll admit that mass can be measured.", since
> the oscillator works, though still does not measure the mass.
--
James R. Frysinger University/College of Charleston
10 Captiva Row Dept. of Physics and Astronomy
Charleston, SC 29407 66 George Street
843.225.0805 Charleston, SC 29424
http://www.cofc.edu/~frysingj [EMAIL PROTECTED]
Cert. Adv. Metrication Specialist 843.953.7644
