The commentary doubles as a synopsis and is reproduced below:

At collieries all over Britain, every precaution is taken to keep out anything that might start a fire underground. For a hundred years now, miners have been using the safety lamp which cannot be opened except in the lamproom, where they're carefully tended after each shift and locked again before being handed out.

Oil lamps have a special use in testing for gas, but most miners use electric lamps nowadays. Even electric lamps can be dangerous if the battery is opened, so every lamp is kept locked by a strong spring. It can only be opened by a powerful magnet in the lamproom.

But lamps have been known to open underground, and so scientists in the National Coal Board have been working to make the locks foolproof. They made the locks safer by making the spring stronger, and the new lock shuts so firmly that they had to design a new, more powerful magnet to open it.

A test of these new locks is being made at Thorne Colliery, Yorkshire, under working conditions. The scientist from Stoke Orchard laboratory demonstrates how to use the new magnet to open the new lock. The head lampman says "let's try the new lock on the old magnet". No, it's not strong enough to open it. The new lock is safe.

Underground workers at Thorne are taking part in these tests. They're using lamps with the new lock which can't open below and start accidents. The success of these trials means extra safety for miners.