He beamed.

Even on the stairs, there was a loud roar. I’ve never liked the noise of the turbine hall, the way it travels through you, making speech impossible. The men on the turbine floor seem to have developed some sort of sign language that I’ve never mastered.

Before we went in, I strapped him into a hard hat, making sure the visor wasn’t too low on his forehead, and I handed him a pair of plastic safety goggles, which he let dangle from his fingers.

‘You have to wear them, Robert. And earplugs. There’s up to ninety decibels of noise in there.’ I showed him how to pinch the foam earplugs between his finger and thumb until they were small enough to be jammed in his ears. Then I put on my own ear protectors and opened the door.

I could feel the noise through the metal grate of the walkway. It came up through my shoes, vibrated along the hairs on my legs. Robert reached for my fingers, his face turned to mine. His green eyes were wide open, unblinking. I squeezed his hand and smiled.

It was thunderous in there. The power station was never silent – there was always a hum or a whirr or a bell somewhere – but the turbine hall breathed noise. I felt a thrill, as I always did, at the size and power of the generator in that great hall: 2,000 milliwatts of electricity were being created here, at this very moment. I often thought of the hall’s long windows and elevated viewing platform as something like a huge theatre, with the generator as the star attraction.

But as we walked together along the viewing grid that ran along the top of the turbine hall, the sound swelling around us, I realised that I couldn’t explain to my son what any of the tin boxes and lagged pipes and bright cables did; even if I’d have shouted at the top of my voice, he wouldn’t have been able to hear a word. We had only sign language, and I wasn’t sure what signs to use to explain the workings of the turbine. So we walked together dumbly while the generator roared around us and the men worked below.

When we were almost at the end, Robert let go of my hand and pointed frantically towards the roof. Following the line of his finger, I saw a pigeon flying upwards. Its wings were spread wide as it glided in front of the long window that stretched from floor to ceiling in that massive hall. Robert kept jabbing his finger in the pigeon’s direction, grinning. I knelt beside him and we watched it together as it flew above the noise of the turbine, way beyond the heads of the men working below.

When we were out of the turbine hall, Robert pulled out his earplugs and said, ‘Does the bird live in there?’

‘Probably. They nest up there sometimes.’

‘Why?’

‘It’s warm and sheltered, I suppose. No one can touch them up there.’