Hans Vogt was a well known German scientist and inventor who as always looking at ways to improve sound recording and reproduction. He had an idea for a new type of microphone without any moving parts, which was subsequently used for many high quality recordings. His novel idea was to coat a length of platinum wire with barium oxide, wrap it round a small bar of ceramic material, and then heat it up by means of a battery. This effectively became a cathode, and the high temperature obtained ionised the surrounding air. Close to this cathode assembly, Hans mounted the small end of a funnel which was given a positive charge from another battery, and thus became an anode. A flow of current could be obtained through the ionised air, and any sound waves reaching the anode would alter the airflow, and with it the current flow to the cathode. These current changes would create small voltage changes across a resistor, which in turn could be fed into an amplifier. He called his microphone the Kathodophone, and in fact it worked far better that the inventor expected. The year? Oh yes, it was 1918!
Hans Vogt went on the develop the traditional condenser microphone, and together with his partners, Josef Engle and Joseph Massolle, designed and built the Tri-Ergon photographic sound recording system which later became known as Tobis Klangfilm.