How isotopes atomic mass was defined?

So we say that 1 Amu (u) = 1/12 of the mass of a Carbon 12 atom.


Im not sure about this, but I believe that first we measured the mass of 1 carbon 12 atom (This includes the mass defects that occur during the formation of nucleus) and then defined that 1 atomic mass unit has a value of 1/12 of the carbon 12 atom mass... Therefore one Carbon 12 atom has **exactly** 12 atomic mass units.


Until here everything is fine.


Now my question is... To define all the other elements **isotopes** atomic mass did we first measured **each** isotope atom mass and then said " If 1/12 of the Carbon 12 atom is equal to X grams and this X grams are equal to 1 atomic unit the then my isotope atom which has Y grams must have Y atomic mass units " did we work it out this way or did we use calculation to define other isotopes mass based on carbon 12 atomic mass ?


Basically what I want to know is, do we need to have the value of an atoms mass in grams or kilograms before we know how many Amu´s it has, or can we calculate it ?




Historically, the relative masses of the isotopes were measured relative to O=16. 


There is a good historical explanation in [A New Mass-Spectrograph and the Whole Number Rule](http://www.jstor.org/stable/94803) (1927). 


At this time (1927), though it was understood that many elements had more than one isotope, only one isotope of oxygen was known. 


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