The wartime aniseed ball timer

Aniseed balls are hard, slow dissolving sweets built up in many thin layers of sugar: see how are aniseed balls made? In 1939 that slow, steady dissolving gave them a secret job in the war.

A magnet in a magazine

In July 1939, Major Millis Jefferis of the War Office read a short article in the popular magazine Armchair Science about very powerful magnets. He contacted the magazine's editor, Stuart Macrae, who offered to design a weapon that would stick to a ship's hull below the waterline. Macrae needed a workshop, so he turned to Cecil Clarke, an engineer who ran the Low Loading Trailer Company in Bedford.

Macrae visited Clarke at home, cleared a number of children out of the living room, and laid out his rough drawings. Work began the next day.

Kitchen bowls and porridge

The first prototypes were homemade. Clarke bought large tin bowls from a nearby branch of Woolworths, a local tinsmith fitted rims to hold small horseshoe magnets, and the bombs were filled with porridge in place of explosive. Clarke, a strong swimmer, tested them in the baths at Bedford Modern School, fixing them to a griddle plate from the family kitchen to stand in for a ship. The weapon was named the limpet mine, after the sea creature that clings to rocks.

The problem of the timer

A limpet mine needed a delay, so that whoever placed it had time to get away. The design used a spring loaded striker held back by a pellet that slowly dissolved in water. Finding the right material was hard. Experts were consulted, but everything tested dissolved at too variable a rate.

The answer came from the children. Aniseed balls belonging to one of Clarke's children were tried, and they worked: each ball was the same shape, and they dissolved in water in about 35 minutes, the only substance tested that dissolved at a predictable rate. The makers, Barratt, could not supply the balls with the holes the mechanism needed, so Macrae drilled them himself. To keep the timer dry until the mine was in place, it was stored in a condom.

Into the war

The first few hundred limpet mines were made in Clarke's workshop, and the first were sent to Yugoslavia and Egypt in the autumn of 1939. Production then passed to outside contractors, and over half a million limpets were made. Jefferis and Macrae became the two key figures of the wartime weapons unit MD1, nicknamed Churchill's Toyshop, and Macrae later wrote a book about it, Winston Churchill's Toyshop, published in 1971.

For the aniseed ball itself, see aniseed balls, and for sweets on the home front, see sweets in wartime Britain

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