# Definition of the Kilogram

I have been reading about how the SI kilogram has been defined throughout history.

From what I understand, the kilogram was initially defined in terms of the mass of a specific volume of water at a specific temperature. However, it is now (and has been for a while) defined as exactly the mass of the International Prototype Kilogram (IPK).

I have also read that there have been efforts to redefine the kilogram through fundamental constants, so that any changes to the mass of the IPK do not affect the value of the kilogram.

My question is: Why not redefine the kilogram in terms of the mass of the volume of water at a specific temperature once again - instead of through a Watt balance or other currently proposed re-definitions?

• What, exactly, is the physics question here? – ACuriousMind Dec 10 '16 at 17:33
• What is the advantage to using a Watt balance to define the Kg as opposed to defining the Kg in terms of the mass of a volume of water at a specific temperature (since the density of water is known)? – Anthony D'Arienzo Dec 10 '16 at 17:34
• @ACuriousMind It's a metrology question. Metrology is a branch of physics. – gerrit Dec 13 '16 at 16:14