Q:

What is the polarity of this barrel plug with a double circle?

Related but doesn't answer my specific icon: How to tell polarity expected of a DC barrel jack?
I have a dell laptop barral plug adapter, and I can't tell from the graphic whether the tip is positive or negative. It looks like the tip has nothing, and there are two outer shells, based on the icon, but I don't think that's how I'm supposed to read it. 
Can someone tell me how to interpret the polarity?

A:

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.

A:

It looks like the tip has nothing, and there are two outer shells, based on the icon

That's correct. The tip is a 3rd conductor carrying some sort of signalling between the adapter and the computer.

