A reference that is old, has no bibtex entry, and probably not well-known throughout the readership. How to cite this?
For cases like this, unless you want to give a historical reference, there's usually no need to cite the primary source, you can just cite your favourite circuit theory book.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?