the early vacuum tubes, which were inefficient, expensive and prone to breaking. His German colleagues told him that the vacuum tubes could not be developed further and that there could be no solution to the problem with the weakness of amplification. Tigerstedt, however, continued to experiment and eventually succeeded in substantially improving on the design of Lee De Forest, achieving much greater amplification than that of the original valves. Tigerstedt also patented his improvement of the triode vacuum valve, which consisted of improving the amplification by rearranging the electrodes cylindrically in the tube, changing the glass mold to prevent vacuum loss and using steel mirrors inside the tubes to prevent electrostatic interference. He had finally solved the most difficult practical problems of talking movies. It is worth noting that his achievement was purely experimental, as there were no prior experiments or mathematical models that he could have followed. Patent for sound-on-film technology and expulsion from Germany This was a major step forward for Tigerstedt, and he was soon able to show a film with sound which was electronically amplified and reproduced through a speaker system. Beginning in 1919, de Forest created his own sound-on-film system, which he called Phonofilm and which used some of Tigerstedt's concepts, along with some of the Tri-Ergon group and of inventor Theodore Case. Tigerstedt was awarded a German patent (number 309,536) on July 28, 1914, for his sound-on-film process. One month later, Tigerstedt was summoned by the German authorities, told that he had been declared an unwanted Russian citizen, and given three days to leave the country. World War I had started and Finland was at that time part of the Russian empire, having been ceded to Russia after the 1808-1809 war between Sweden and Russia. As another consequence of the declaration of war between Russia and Germany, the German government invalidated all German patents held by citizens of enemy nations. Ultrasound experiments Tigerstedt moved to Denmark, where he continued working on film sound recording devices at the Petersen & Poulsen company. He befriended the Danish inventor Valdemar Poulsen (1869–1942), who was the inventor of the telegraphone steel wire magnetic recorder. Tigerstedt, too, had experimented with recording sound on a steel wire. He attempted to transmit speech across the Jutland straits using 30 kHz ultrasound, but the results were not promising. He also experimented with transmitting sound underwater, which was more successful, and he was