I have to be disciplined, to keep both things going. It would be easy to let one or other slip. The secret is to never put off to tomorrow what I can do today, and to keep to a routine. When I get home I do any marking and planning first, and then, after a little break, disappear upstairs and start tapping away at the keyboard. I usually do two to three hours, though if the muse strikes I’ll be there into the night. My wife is understanding; she can see that it keeps me out of mischief!

The writing came about because of a website I set up called Creative Chemistry. It started off as a small thing, intended just for my pupils, but we kept adding more features and it soon built up a following of users across the country. Someone at Heinemann saw the site and approached me about doing a textbook. One thing led to another and before I knew it I had been commissioned to write a series of 12 books about the periodic table.

The BBC also saw the website and asked me to produce material for its Bitesize revision site. I’ve reached the point where I’m having to turn down work.

The extra money is useful, but I’m not sitting at home surrounded by piles of pound;5 notes. I suppose my writing work means my total income is similar to that of a senior manager in business. I recently sold the website, because I no longer had time to maintain it. I was able to get a reasonable sum for it, the kind of money that would pay for a nice holiday.

In fact, most of it went into the bank; we’re not big spenders. But it’s nice to be able to send our children on any school trips, or to know that if the fence blows down we can afford to get it fixed. Sadly, not everyone who is living on a teacher’s income can say that.

I’ve always tried to earn a little extra over and above my salary. When our children were young, I used to teach at the Open University summer school.

I wasn’t paid a king’s ransom, but it was useful experience. When you have six weeks’ summer holiday, you don’t mind losing a few days. I’ve also marked key stage 3 Sats, and GCSE scripts. Again, it’s a useful thing to do at a particular stage of your career. I used to mark about 500 scripts. The first 100 are slow because you’re getting used to the mark scheme, the next 300 are fine, and the last 100 you would willingly pay someone else to take off your hands. I also tried moderating, but only for a year. It was a good way of learning what other schools were doing, but I found it time-consuming and more complex than marking scripts.

Now, the books and CDs take up all my spare time. Fortunately, my headteacher is supportive. He can see that what I’m doing reflects well on the school. There’s also a definite synergy between my two areas of work.

The fact that I’m a practising teacher appeals to publishers, while there are many things which occur to me when I’m writing which I am then able to use in my own lessons.

If my other interests started to get in the way of my day-to-day teaching, then I’m sure my head would take a different view. After all, he’s paying me to do a job and has a right to expect that I do it to the best of my ability. Teaching is certainly my priority, and I wouldn’t want to give it up and write full-time. But I’m fascinated by science and if I can teach it and write about it as well, then that’s just perfect for me.

Nigel Saunders is head of chemistry at Harrogate Granby high school