By the autumn
of 1904, the company's scientific adviser, Professor J. Ambrose Fleming, was
becoming increasingly deaf, which made it difficult to identify the Morse code
signals relayed from the magnetic detector to his earphone.
In seeking a visual alternative, he recalled an odd phenomenon first noticed and patented by the American inventor of incandescent lamp, Thomas Edison. Edison noticed that when the bulbs of his early lamps blackened with use, the positive side of the filament cast a 'shadow' in the blackening of the bulb. He inserted a metal plate in one of his bulbs and found that a current would flow if the plate were made positive with respect to the filament but not if the polarity was reversed. This effect was known as the 'Edison Effect'. When Fleming visited the USA in 1889, he met Edison who gave him some of these special lamps. Fleming returned to England and repeated Edison's experiments and then put the lamps away in a laboratory cupboard and forgot about them.
Recalling these experiments, he retrieved the lamps and set up an oscillation circuit with a Leyden jar and driven by a spark coil. He then made up a similar circuit a few feet away incorporating one of the lamps, a condenser and a mirror galvanometer.
When the spark coil was excited, Fleming observed that the mirror galvanometer gave a large and steady deflection showing that a unidirectional current was passing. This bulb fitted with a metal plate was acting as an 'oscillation valve' or rectifier.
"I have," he wrote to Marconi, "found a method of rectifying electrical oscillations - that is making the flow of electricity all in the same direction so that I can detect them with an ordinary mirror galvanometer." With the galvanometer and his one-way valve, he could receive signals on an aerial and "measure exactly the effect of the transmitter".
He added: "I have not mentioned this to anyone yet as it may become very useful."
Marconi put the valve into production immediately. What neither he nor Fleming could then know was that it would eventually lead radio from Morse clicks into speech and full sound broadcasting - and into litigation that cost as much money as it ever earned.
In seeking a visual alternative, he recalled an odd phenomenon first noticed and patented by the American inventor of incandescent lamp, Thomas Edison. Edison noticed that when the bulbs of his early lamps blackened with use, the positive side of the filament cast a 'shadow' in the blackening of the bulb. He inserted a metal plate in one of his bulbs and found that a current would flow if the plate were made positive with respect to the filament but not if the polarity was reversed. This effect was known as the 'Edison Effect'. When Fleming visited the USA in 1889, he met Edison who gave him some of these special lamps. Fleming returned to England and repeated Edison's experiments and then put the lamps away in a laboratory cupboard and forgot about them.
Recalling these experiments, he retrieved the lamps and set up an oscillation circuit with a Leyden jar and driven by a spark coil. He then made up a similar circuit a few feet away incorporating one of the lamps, a condenser and a mirror galvanometer.
When the spark coil was excited, Fleming observed that the mirror galvanometer gave a large and steady deflection showing that a unidirectional current was passing. This bulb fitted with a metal plate was acting as an 'oscillation valve' or rectifier.
"I have," he wrote to Marconi, "found a method of rectifying electrical oscillations - that is making the flow of electricity all in the same direction so that I can detect them with an ordinary mirror galvanometer." With the galvanometer and his one-way valve, he could receive signals on an aerial and "measure exactly the effect of the transmitter".
He added: "I have not mentioned this to anyone yet as it may become very useful."
Marconi put the valve into production immediately. What neither he nor Fleming could then know was that it would eventually lead radio from Morse clicks into speech and full sound broadcasting - and into litigation that cost as much money as it ever earned.
