the explosion would be focused on a small area and would rupture an armoured plate. Sappers call such a device a "poultice" or "squash head" charge. Jefferis enlisted Bauer and Schulman of the Colloid Science Department of Cambridge University, they experimented with lengths of bicycle inner tube filled with plasticine to represent the explosive. These were fitted with wooden handles and dipped in rubber solution to make them sticky. In experiments, these prototypes proved difficult to aim and only stuck to the metal bins used to represent tanks. With the end of the Battle of France and the evacuation of the British Expeditionary Force from the port of Dunkirk between 26 May and 4 June 1940, a German invasion of Great Britain seemed likely. However, the British Army was not well equipped to defend the country in such an event; in the weeks after the Dunkirk evacuation it could only field twenty-seven divisions. The army was particularly short of anti-tank guns, 840 of which had been left behind in France and only 167 were available in Britain; ammunition was so scarce for the remaining guns that regulations forbid even a single round being used for training purposes. Under the circumstances, Jefferis considered that his idea might have more general application for the British Army and the Home Guard. Jefferis was in charge of a department known as , which had been created to develop and deliver weapons for use by guerrilla and resistance groups in occupied Europe. was now charged with the development of the sticky bomb. The problem of designing a sticky bomb was delegated to Robert Stuart Macrae. Some sort of flexible bag was required to contain an explosive gel so that it did not matter how the bomb landed on the target. However, a flexible bag was found to be difficult to throw and trials had not been satisfactory. Discussions of the problem in Macrae's office were overheard by Gordon Norwood, a master printer who Macrae had recruited from his former magazine publishing employer, but he was not directly working on the weapon. Norwood suggested that what was needed was a frangible container. He obtained a 150 W light bulb with which to demonstrate his point: a spherical glass flask inside a sock of woven wool, rigid when thrown, but on contact the glass broke and the bomb deformed to the required shape. Experiments with glass flasks