This weekend I filled an important British culture gap: I watched Porridge, the cult series set in a prison, for the first time. I also finally learnt why prison wardens are called screws. The explanation Gabrielle gave me (some kind of torture instrument with a screw allowing wardens to make it even harder) seemed very far-fetched, so I checked it on the Internet. A page showing examples of hard labour in a Scottish prison confirmed my friend’s theory (with photos):

The crank machine, a form of useless labour, was introduced in the middle of the last century to make prison life tougher for those prisoners sentenced to hard labour. Male prisoners had to turn the handle 14,400 times a day.
The number of revolutions were registered on the dial. The warder could make the task harder by tightening a screw, hence the slang word for prison warders – "screw".