Hello, all,
I reread the recent discussion (as of Aug. 28) of charge conservation and realized that the close topic of charge invariance may be also relevant, see Sec. 6.3 of my book "Special Relativity and How It Works". It shows some pitfalls even in the simplest (albeit highly idealized) model of DC in a single straight wire. That may be of interest when teaching the corresponding issues in a graduate or advanced undergraduate course.
I will appreciate any comments and use them if it comes to the next edition of the book.
Moses Fayngold,NJIT