Source: Lessons In Electric Circuits—Volume I
Example:
Since air is an insulating material and an air gap separates the two pieces of wire, the once-continuous path has now been broken and electrons cannot flow from Source to Destination. This is like cutting a water pipe in two and capping off the broken ends of the pipe: water can't flow if there's no exit out of the pipe. In electrical terms, we had a condition of electrical continuity when the wire was in one piece and now that continuity is broken with the wire cut and separated.
I know of the idiomatic expression to cap something off, of course, but it is obviously not this idiom that's being used here. The one in question looks more like a typical phrasal verb. But what do you actually need to do physically to cap something off like the end of a plastic water pipe, for example? What does the action look like?
Best Answer
Capping something off means putting a cap on it, whether a metaphorical cap (like capping your career off by becoming the president of your company) or a literal, physical cap such as the cap you put on the end of a pipe.
There are many sorts of pipe cap, of many different materials. Some screw onto a threaded pipe, others are simply pushed on and held in place by friction or by clamps. What sort of cap you use will depend on what sort of pipe you're working with and how much pressure has to be resisted. Here's a typical threaded cap on a water pipe: