In a regular household plug-in, there are 3 prongs: positive, negative, and ground. But, when doing wiring projects, many times they refer to the negative lead as the ground. Why is it that this lead is sometimes referred to as the ground, while other times, the ground is a completely separate wire altogether? This seems like a contradiction that irks me to no end. For example, today in lab, I was to hook up the oscilloscope. There were 3 available ports: a red one, a black one, and one labeled ground. I had assumed the circuit needed to originate from the positive red port and conclude at the negative black port; however, I was told this was incorrect. I thought the 'ground' was a just-in-case-of-an-overload route for excess electricity to disperse away from the circuit, as in the case of a household plug-in. But, now it's changed to include the negative terminal, but only "sometimes"? What's the deal?