The commentary doubles as a synopsis, and is reproduced here in full:

Two years ago, Gerald Russon won the NCB East Midlands safety competition with this coal face conveyor signal. Mr Russon is chief electrician of Thoresby Colliery near Nottingham, where the conveyor signal was first put into use.

Any new idea needs talking over with the people who are going to use it. Mining is an old industry, and custom plays a big part underground. So the men at the coal face get a first-hand explanation.

The device is simple. In the electrical workshop, Russon shows the men who will erect it in the pit just how it works. One pull on the communication cord - there's no five pound fine - and the conveyor motor stops at once. The motor man waits for a two-bell signal on the wire before he restarts the conveyor.

Underground, they've built the system at the coal face. Now that the face workers are used to it, they have an extra safety device. No more shouting or whistling to the motor man a hundred yards up the conveyor. If anything goes wrong, a quick tug on the wire and the danger is removed.

The idea has been so successful that nearly all East Midland pits are installing it at their coal faces, and at Thoresby they plan to put it in on every trunk conveyor throughout the pit.