I hate to be a pedantic twat, but I'm afraid the above isn't right. Bell entered the fray at half time, and for half time subs in those days, they checked the studs before the teams came out so the sub just ran out with the rest of the team. Unlike when a sub came on during play, at half time there was no card displayed by the physio. Commentators used to have to try to look round the teams as they lined up for kick off to spot whether there were any changes.
The way it actually unfolded is that, as the team came out at the start of the second half, a roar went up in the middle of the Kippax because, being directly opposite, they could see him in the tunnel before he emerged onto the pitch. And that spread right round the ground as more and more people realised that Colin Bell was coming on. It turned into a huge, rapturous ovation from literally everyone there (I didn't see anyone round me not joining in). To be fair, the Geordies applauded and cheered him as well.
It went on for several minutes, but even when it subsided the atmosphere for the whole second half was incredibly charged and the team responded. Newcastle were a really poor side that season (they went down, winning only four league games in the entire campaign) and had showed it in the first half, but we'd looked no better. Once Bell had made his entry, the atmosphere spurred us on and we looked like scoring every time we attacked.
I've watched a lot of football in my time, including many non-City matches in different parts of England and in various locations abroad. I've never experienced another atmosphere remotely like the one at Maine Road that afternoon.