Inside of the plug/outer barrel? Answered
First of all, thank you very much for offering this nice class!
Can I ask you a quick question?
What did you mean by 'inside is a positive DC voltage and the outer barrel is ground' here: "Aside from voltage and current, if you look carefully at the bottom of the label, you will see an illustration that even tells you the inside of the plug is a positive DC voltage and the outer barrel is ground."
Once again, thank you very much!