Developments

The history of the vacuum tube continues with the work of a prolific scientist: John Ambrose Fleming. Like Edison, Fleming was also fascinated by the effect and started to perform experiments around the idea. For example in 1889 he had some bulbs made up for him by the Ediswan Company in UK. Using those bulbs he reproduced the Edison Effect, using a steady state charge.

A few years later he had the idea to use alternating potentials within this setup. He observed that if an alternating current with a frequency between 80 and 100 Hz was passed through the bulb, then only one half of the cycle was passed. In other words it was rectified to produce a direct current. In those times there was a lack of understanding about the operation of the device. Therefore no further progress was made.

In 1897, Sir Joseph Thomson discovered that atoms were made from even smaller particles, one of which was negatively charged: the electron. Accordingly it was quickly realized that it was electrons that were being emitted from the heated filament in the bulb. This also explained why they were attracted to an electrode with a positive charge.

A great chapter of the history of the vacuum tube: Fleming’s valve

history of the vacuum tube - Fleming's diode

In addition to his work at University College London, Fleming was also a consultant to Marconi. Marconi was at this time rapidly increasing the distances over which wireless signals could be used for communication. For example, in 1901 he made the first transatlantic transmission. Seeking to improve the quality and distances in wireless communications, Fleming saw that the major limitation was the sensitivity of the receiving equipment. This was caused by the poor sensitivity of the detector. At this time magnetic detectors were used, and those were very inefficient.

Fleming was searching ways of improving this, and in November 1904 whilst he was walking along Gower Street in the West End of London, he had what he called “sudden very happy thought”. He wondered if the Edison Effect could be used to rectify what he called the “feeble to and fro motions of electricity from an aerial wire”. Fleming set up an experiment and was able to prove that his idea worked.

Concept of the diode vacuum tube

Fleming called his new invention an “oscillation valve” because it acted in a similar way to a valve in a pump that allows gas or water to move in only one direction. He patented the idea that was clearly a major step forwards in wireless technology. Even though the vacuum tube was still in its infancy it was a major improvement over the coherer or magnetic detectors that were available at the time.

Despite its clear advantage over other detectors, Fleming’s oscillation valve or vacuum tube was not widely used. Valves or tubes were difficult and expensive to make. Their heaters consumed large amounts of power and this had to be supplied by expensive batteries. Additionally some cheaper devices were discovered in 1906. Devices that were forerunners of the Cat’s Whisker detectors that were used in crystal sets until the mid-1920s appeared. Those devices had many limitations but they were very much cheaper than Fleming’s oscillation valve and as a result they were quickly adopted.

to be continued.