Cartridges in general

The MM thread is in decline because the driving force Raul left his own thread. Besides the thread become to large so not easy to find whatever. Meanwhile J.Carr made the suggestion to extend the thread such that it would include electret , electorstatic and optical kinds in addition to the usual MC and MM kinds. J.Carr is invited to give some
kind of introduction to those 'strange kinds' of carts as the start for this new thread.