For some reason the card expires. You have to use its balance up, go to an office to have the balance put onto a new card or, if like me, you live outside Chicago and come on weekends, you can ask for reimbursement. Unfortunately, it came in the form of magnetic strip cards. I am not sure how it will work on PACE and of course, I doubt I can transfer for $0.25.
However, this way I was left with a non-functional card. Take a look what is inside: an electronic chip and an antenna.
The chip is on the right upper side, which probably explains why some people with a worn card have still good luck using the card by touching that corner to the scanning disk on a bus. The "worn" card probably means that it was bent and the copper wires forming the antenna inside the plastic were overstretched and broke.
What you see above is a card and below it is what was left after the plastic of the card was dissolved away by an organic solvent.
GSM SIM Card un-encapsulated
The same process gives a look inside the GSM SIM Card.
The liquid crystal, under the green 100% - to - red would be heated intermittently a a
resistive film, when the white dot on left, an another (not visible) at the bottom were
pressed. The resistive film was a narrow triangle, so it heated more at its narrower
section (under the red). A weak battery would thus heat only the narrow part enough for
the liquid crystal to turn milky and indicate state of charge. Fully charged battery would
supply enough to heat the who strip and indicate higher current capability of a fresh
8 May 2009, SIM Card added 6 January 2012