NIST Wants An Electronic Kilogram

Dearing writes "According to the Global Engineering Journal, NIST, those not-so-standard standards people, want to give up the hunk of metal they've been calling a kilogram, even though it never weighs the same twice. In it's place, an electronic kilogram could act as the permanent standard."

Comments

Of course it never weighs the same...

[Robber Baron]

A kilogram is a unit of mass not weight. Weight is dependant on gravity. Mass is not.

I visited NIST and had it explained

[dragons_flight]

IIRC the idea is to convert the standard of mass to a number of electrons accelerated by some well known voltage.

The electrons since they are moving, produce a magnetic field which pushes against a well known reference magnetic field (which can be measured without concern for mass). This magnetic repulsion is used to balance a 1 kg reference mass against gravity.

Since gravity produces acceleration independant of mass (ma=F=mg => a=g), it's also possible to measure the local gravity to a high precision by means of the acceleration with needing to know something's mass.

Thus we have a way define mass in terms of a number of electrons (and a geometry of the path they take, technically) and other measured quantities which don't use mass in their standards.

You could say mass is so many atoms of some reference substance, but how do you measure it? Since you can't first weigh it and extrapolate from there. Similarly volume would depend on temperature, structural arrangement, and other things. The people at NIST claim this provides a more easily reproducible method of defining mass. (Of course I'd rather just stick with the electronic scale or balance pan since these tend to be accurate enough for me.)