How to cite scientific laws There are several laws that many people know verbatim and could quote off of the top of their heads. Take Gay Lussac's (The Pressure) Law for instance:


  The pressure of a gas of fixed mass and fixed volume,is directly proportional to the gas's absolute temperature.


Which is the wording that is used in many textbooks and on Wikipedia. However, in many instances, this included, it can be difficult to locate the original paper where the law was defined due to the passage of time. A similar example of this could be Pythagoras' Theorum:


  The square of the hypoteneuse is equal to the sum of the squares of the other two sides.


So my question is thus, how should one go about citing these definitions? Should we just assume that everyone is aware of their provenance due to them being universally understood laws in academic circles and thus they can stand on their own axiomically?

If it's relevant, my university conforms to their own variant of Harvard referencing.
Common knowledge doesn't need to be cited. The threshold for what counts as "common" varies: My rule of thumb for a scientific publication is that if it is taught in the intro lectures of your discipline, it's common knowledge. I sometimes mention Planck's law at the start of a Methods section, and don't give a citation. Exceptions are of course if your paper is about the history of the law itself, or a related topic, in which case you would be interested in specific historical formulations and usage contexts anyway and should cite a source. (Or for example in the pedagogy of physics, if you want to compare textbook formulations - cite them.) Another case is if you use something that's common knowledge in a different discipline. I find this to be the case with statistical and machine learning techniques which a reader in my (geosciences) area might not be familiar with, but which would need no citation in a statistics paper. In this case, I found a widely used textbook to cite (with "e.g." before the reference).

References have a purpose. You can ask yourself: Who is my reader, and would they either know this or recognize it as background knowledge that they can look up in standard basic references? Pythagoras theorem - taught to every teen in the country, so needs no cite. 

For a class paper, you should probably ask the instructor about where the limit for cite-worthy lies. I've found that "what was taught in THIS class and its immediate predecessors" is usually considered common knowledge. If you want to cite something, a standard textbook will do.