20101
Physics, Astronomy & Cosmology / Re: Why is NIST basing the new kilogram (kg) definition on gravity?
« on: 02/12/2017 09:59:48 »Will the watt balance give the same definition value of 1kg when performed under different gravitys?
Yes, it does.
Essentially, you get a coil of wire on a balance and a magnet set on the ground
You set the thing swinging- it doesn't matter how, but gravity is the easiest way to do it, and you measure the voltage induced in the coil. That gives you a measure of the strength of the field from the magnet.
That expression includes g in the calculation
Then, you put the "kilo" on the balance and measure the current needed to bring the balance back to "level".
That current depends on the force produced by gravity on the kilo and on the strength of the magnetic field.
The force also depends on stuff like the number of turns and the size of the coils and lots of other stuff that's hard to measure accurately (you may thing it's easy to count the turns in a coil- but how do you account for the wires feeding current to an from the magnet?)
The really clever bit is that all those thing affect the voltage produced when you move the coil through the field of the magnet in exactly the same way that they affect the current needed to give a particular force.
A lot of the "difficult" terms cancel out; unfortunately, g isn't one of them.
So, the value of g needs to be measured in order to calculate the mass, but measuring g is "relatively easy".
https://en.wikipedia.org/wiki/Gravimeter#Absolute_gravimeters