Personal Computer Self Centring Joystick - Dismantled for reference and curiosity
The Apple II Joystick is an analogue input peripheral primarily used for playing games.
The joystick connects to the computer using a DE-9 connector and while there is a green wire present in the lead it is not used.
Interestingly the joystick packaging states that it can be connected to an Apple II computer, however this was only possible with an adapter. Even though Apple stopped making the adapters they could still be purchased from Southern California Research Group. The same company also made adapters that allowed two joysticks to be connected to an Apple II+ and an Apple IIGS.
Analogue joysticks use a continuous current through two potentiometers (pot(s)) that varies as the user moves the stick. This allows for gradual increases in movement VS digital joysticks having usually only four on / off contacts for up, down, left and right; combinations of the four create diagonal input. Modern gaming thumb sticks are analogue VS the D-Pad which is digital.
The joystick was intended to be self centring meaning that if you released the stick it would find a middle location through the use of loaded springs and movement output would stop. Non-self centring joysticks just flop about. This centre is the dead spot where the software and computer hardware accepted that this level of current from each pot meant the joystick was in the middle so no action should be taken.
In reality this centre could be slightly off and so there were two controls on the bottom of the joystick to allow the user to set the centre.