He was in his late fifties, always wore a white laboratory coat which had more holes in it than cloth, and used to walk slowly around the corridors with his glasses perched on the end of his nose.

The great thing about the man was that he taught you the processes of logical thought. In retrospect, I realise that his fundamental principle was simple: if you know how to think properly, you can understand anything.

During lessons he would point at you and ask “What happens when you add such and such a chemical to so and so?” When you had answered, he wouldn’t say “Right” or “Wrong”, he’d just shake his head sadly and say:

“Well, I wouldn’t know”.

That was a signal that whatever conclusion you’d arrived at, you hadn’t gone through the right processes to get there. “Well, I wouldn’t know” became a bit of a catchphrase around the school - we used it to reply to any old question from a friend, such as “Are you going to catch the bus?” Like a lot of teachers of that era, Alf ruled largely by fear -and no one dared ridicule him or muck about in his classes. If he threw an angry glance in your direction, and reprimanded you in that hard-edged Lancashire accent, you wanted the ground to open up and swallow you. The most common sound in the school was the head’s slipper hitting someone’s backside, so you knew you risked corporal punishment (though I escaped it).

I think Alf was probably not a man of great patience, especially with O-level classes. But when you got to the sixth form and were more mature in your attitude to chemistry, he mellowed considerably and was much more encouraging.

He was a funny old devil when it came to materials in the lab. One experiment we did a lot involved adding hydrochloric acid to calcium carbonate, and the lab assistant was always ordering the latter from the school’s chemical suppliers. This angered Alf, because every day lorries from the limestone pits in Derbyshire would come thundering past outside the school, laden with calcium carbonate, and shedding chippings on to the road which Alf would go out and collect, to save money.

I remember one occasion in the lab with him, when we were talking glibly about radioactivity. He paused, looked up and said: “Doctors think they know everything about radioactivity. They think they can blast away cancers with it, but sometimes they just blast them over a wider area.” His wife had died a couple of years earlier, after radiotherapy. That remark of his was very moving.

Our chemistry lessons were divided between qualitative analysis and quantitative analysis. The latter was more mathematically-based, and I struggled with it. The qualitative stuff was almost like a game show: you’re given some unidentified powder and have 35 minutes to discover what it is by doing a logical series of experiments. Alf was good at coming up with chemicals that would throw you off the trail.

With Alf we used traditional chemical balances - scales with weights so tiny that you had to lift them with tweezers. Younger teachers would encourage us to use electric balances that would just give a readout, saying 2.6854 grammes, or whatever. This infuriated Alf. “How will you ever understand what weight and mass really mean if you can’t see material on one side and weights on the other?” He was absolutely right.

He was equally intolerant of the way the chemistry syllabus was changing. He used to hand round theA-level syllabus with the words he thought based on airy-fairy notions underlined. The one he objected to most was: “Pupils should have an understanding of the concept of entropy”.

To Alf, that was like writing a philosophy syllabus with a throwaway line at the bottom that says “Pupils must be aware of every philosophy in the world”. He thought this notion had no place in a school syllabus and when we went on visits to Sheffield University, he’d argue the point endlessly with their chemistry professor, George Porter.

I managed to get a B at A-level. I’ve forgotten the facts I learned then, and I don’t remember Alf with fondness, but with my own children going through secondary school I’ve come to realise that I couldn’t have had a better teacher.

Whatever problems I encounter, in my broadcasting, my writing, or when I was a farmer, I will go through them in a manner Alf would approve of. I can hear his voice over my shoulder, encouraging me to do things in a logical way.

I was invited to his 90th birthday party a couple of years ago, but no one from the school knew where I lived and, much to my regret, the invitation was sent care of a newspaper office that didn’t forward it to me. I would love to have gone.

Paul Heiney, 48, writer, broadcaster and a former farmer, has appeared on numerous radio and television programmes, including ‘That’s Life’. His latest novel, ‘Golden Apples’, is published by Hodder and Stoughton (pound;16.99). He was talking to Daniel Rosenthal