The outer cylinder is negative with the inner surface being positive.
The tip connects to a Maxim One-Wire memory in the power supply that is read by the computer to obtain information such as power supply capability, serial number etc.
Be VERY careful if you probe the cable, if you accidentally short the centre pin to the inner cylinder it may destroy the One-Wire memory (on the one I investigated there was NO protection against this!).
If the memory is non-functional the computer will still be powered but it will assume it is a non-Dell low-power supply, it may reduce the speed of the computer to minimize power consumption and may refuse to charge the battery.
I accidentally damaged my power supply when I probed a Dell power supply to see what the voltage was. There is not really a way to repair the power supply if this happens.