He was on holiday in Germany the next summer when World War I broke out. All British people who were in Germany were sent to the Ruhleben P.O.W. Camp just outside Berlin. The place where the detainees (prisoners of war) were held had been a horse racecourse. There were lots of stables, and in each stable there were 27 horse boxes which became the detainees' home during the war. The detainees were allowed to do lots of things to keep themselves busy. They could get books, and Ellis made good use of his time to study. Another detainee in the camp was the scientist James Chadwick. Chadwick taught Ellis a lot and together they made a laboratory in one of the horse stables where they made scientific experiments on the photochemical process.