A reference that is old, has no bibtex entry, and probably not well-known throughout the readership. How to cite this? I am writing an article for an audience that mostly consists of computer scientists. A specific part of the subject is motivated by Kirchhoff's laws. I intend to mention this fact and reference the rules. But that is easier said then done. Apparently, Kirchhoff predates referencing.

How to reference a source that is so old, it has no bibtex entry, but is also probably not well-known  throughout the readership?

edit: I want to clarify, that I can also imagine to not reference it at all, but I find it difficult to draw a line here. Can I assume that every reader is familiar with a term? For instance, can I omit a citation to Damas/Hindley/Milner when introducing ML to modeling engineers? How about Newton's method for computer scientists?
For cases like this, unless you want to give a historical reference, there's usually no need to cite the primary source, you can just cite your favourite circuit theory book.

Anyway, if you really wish, you can certainly cite the original paper too, which can be found, e.g., here.

For what concerns the added question on where to draw the line, you can have a look at the following question, and the answers therein: How generous should I be with citations?