From the Audion to talking movies

Electrical Tester – 25 September 2017

Author: Keith Wilson, Electrical Engineer

Although a Yale graduate and a doctor in Physics, de Forest was not primarily a physicist, but an inventor. After spending the first few years of his career working on the wireless telegraph – an invention patented by Marconi a decade before – de Forest grew tired of the limitations of the wireless technology of the era and began to experiment with novel ideas that led him, in 1906, to register a patent on his ‘Audion’ - a vacuum tube he crudely described as a ‘detector of sound’.

An ever-restless inventor, de Forest went on to design, a year later, an arc-based radiotelephone transmitter and the Audion receiver. His objective was to transmit music live from one city to another and potentially to improve transatlantic communications.

But what exactly was this mysterious Audion? It was, in fact, nothing more than a triode valve. The idea for it came to de Forest when he noticed that the gas light in his laboratory dimmed every time his spark telegraphy apparatus was on, and that the light returned to full brilliance once the telegraph was off.

He believed that he might be able to exploit this effect to produce a new kind of detector for wireless signal, and that he would achieve better results if the gas was shielded from air currents. So he made a glass vessel partially filled with gas and containing two separate electrodes. He then heated the electrodes electrically and exposed the tube to radiation. Unhappy with the results, he tried to achieve greater sensitivity for his detector by adding a third electrode to the tube. This initially took the form of a pierced platinum plate and later a piece of platinum wire in a zig-zag shape, which de Forest called a “grid”, supposedly because of its passing resemblance to the grid of an American football field.

His tests proved that by varying the potential difference between the grid and the heated electrode or cathode, the flow of electrons between this cathode and the anode could be controlled. However, de Forest’s early devices were exceptionally temperamental, not least because he insisted that a certain amount of residual gas needed to be left in the tube to ensure that they would work. As a result, uptake of the Audion was slow.

De Forest also became embroiled in a patent dispute with the Marconi company over his new invention, as this company claimed that the device infringed Fleming’s patent for the diode valve (vacuum tube), which by this time it had purchased.

Yet de Forest’s creative genius was not daunted, and he began researching wireless telephony. His endeavours proved successful and in 1907 the US Navy purchased from him an arc transmitter with a vacuum tube detector. The first order was for 27 sets of these new devices, which were going to be used for military manoeuvres in the Pacific Ocean.

This newly found commercial success allowed de Forest to keep his eyes on the big prize – transmitting live music. He first trialled the transmission of Caruso’s voice during a Metropolitan Opera Company concert. His efforts received mixed reviews and some observers were unimpressed with the quality of sound received.

Undeterred, de Forest continued his work on the Audion. His researches revealed that the tube, when properly configured, could be used not only as a detector but also as an amplifier and an oscillator. This discovery eventually led to the abandonment of Marconi’s spark apparatus, Poulsen’s arc transmitter and Alexanderson’s high-frequency alternator, all of which came more into the category of very costly “fixed plant” rather than the compact and relatively inexpensive transmitters made possible by the Audion.

Remaining an inquisitive mind till the end of his days, de Forest focused in his later years on the ‘talking movies’ technology that was just emerging at the time. Between 1920 and 1930 he invented, patented and improved the system of recording a sound track on a strip of film, thus allowing accurate and reliable synchronization of sound and picture. Basically, Lee de Forest invented the talking motion picture.

With 216 US patents achieved throughout his life, de Forest is now considered by those who know of him an outstanding inventor, but he has never gained the fame an public recognition accorded to, for example, Edison and Marconi. Nevertheless, he has been described by some as the “father of radio” and the “grandfather of television.”

His endeavours in the field of three-electrode valve technology fully support this recognition and it would also be fair to say that he played a pivotal role in the early days of the whole field of electronics. In fact, for his achievements he was proposed, with strong backing, for the Nobel Prize for Physics, but was ultimately unsuccessful. Should he have received the prize? You must be the judge of that!