There are also magnetic sensors that look almost identical and they will detect either a N or S pole from a short distance just like these inductive sensors. They come stock with the inexpensive tachometers that I've installed on all my equipment:
Thank you sir. I have one of those and have lost the instructions with the hookup data. Now I can install it as soon as I do the 50 or so things in backlog. I have been working on a house to sell lately and it has been eating up all my time.