Now that's a calculator!

I remember my brother building this Sinclair Scientific calculator many, many years ago. Powered by 4 x AAA batteries and operating with reverse polish notation. I used it in earnest doing my HND at Southall College of Technology. And still working! Both me and the calculator.

Battery life better than expected as people who borrowed it quickly returned it when they couldn't find the 'equals' key. And there isn't one.