The modern era of electronics began with the invention of the light bulb. But, contrary to popular belief, it wasn’t the bulb’s ability to produce light that changed the course of history. Instead, it was a curious observation made by Thomas Edison that paved the way for an electronics revolution.
|1879||Early light bulbs||Consisted of a carbon filament sealed inside a glass bulb with a vacuum inside; when a potential difference was applied across the filament, current flowed through it, heating it up to over 2000 Kelvin so hot that it glowed|
|1883||Edison’s observation||Noticed that over a bulb’s lifetime, the glass became discolored, turning yellow and then brown, but only on one side|
|1904||Fleming’s thermionic diode||Patented a device that was very similar to Edison’s light bulb but with a second electrode in the bulb; by charging this plate positively with respect to the filament, electrons could be accelerated across the gap completing the circuit|
|Early 1900s||Amplification problem||The big problem in electronics was amplification; radio had just been invented but its range was limited by the lack of reliable equipment that could boost weak signals; similarly, telephone calls were limited to at most 1300 kilometers because by that point, the signal was too faint to hear|
|1906||De Forest’s triode||Took the diode and added another electrode into the bulb; this electrode wasn’t a solid piece of metal but rather a sparse wire mesh, and it was positioned in between the filament or cathode and the anode; with three electrodes, it was called a triode|
|Mid-1900s||Vacuum tubes||The first practical vacuum tube device was created by combining a few diodes and a capacitor; this led to a fairly steady direct current, and the vacuum tube became the model for all vacuum tubes that would dominate the industry for the next half-century|
The Early Light Bulb: A Vacuum-Sealed Carbon Filament
The first light bulbs consisted of a carbon filament sealed inside a glass bulb with a vacuum inside. When a potential difference was applied across the filament, current flowed through it, heating it up to over 2000 Kelvin, causing it to glow. However, if there had been much oxygen in the bulb, the filament would have burned immediately. Therefore, the vacuum prevented this from happening.
The Edison Effect: The Emission of Electrons
- Edison observed that over a bulb’s lifetime, the glass became discolored, turning yellow and then brown, but only on one side.
- The heated filament emits not only light and heat but also electrons.
- This phenomenon is known as thermionic emission.
- These electrons floating around were unobstructed since they were in a vacuum.
- However, since there was a potential difference across the wires that led to the filament, the electrons were attracted to the positive wire.
- Most of the electrons would whiz straight past the wire and crash into the glass over time, discoloring it only on the positive side.
- This observation became widely known as the Edison effect.
The Thermionic Diode: A One-Way Street for Electricity
In 1904, John Ambrose Fleming patented a device that was very similar to Edison’s light bulb, but with one important addition: a second electrode in the bulb.
By charging this plate positively with respect to the filament, electrons could be accelerated across the gap, completing the circuit. However, if the plate were slightly negative relative to the filament, then it would repel electrons, and no current would flow.
Fleming called his device a “one-way street” for electricity, since only one of the electrodes was heated. Electrons could only flow from there to the plate and not the other way around.
The device was called a thermionic diode, and it was used initially for detecting radio signals. But it could also convert alternating current to direct current.
The Triode: High Frequency Amplification
- Scientists improved the design by placing the filament in the center and the anode as a cylinder surrounding it.
- This design captured more of the electrons and allowed for larger currents to flow.
- The big problem in electronics at that time was amplification, which limited the range of radio signals.
- Lee De Forest added a third electrode, a sparse wire mesh, positioned between the filament or cathode and the anode.
- With three electrodes, the new device was called a “triode.”
- A small change in voltage on the grid could now control a huge voltage at the anode, allowing for high-frequency amplification.
In conclusion, the light bulb was the unsung hero of the electronics revolution. It was not the ability to produce light that changed the world, but rather the discovery of the emission of electrons. The thermionic diode and the triode were two of the most significant inventions that allowed for amplification and conversion of AC to DC. These devices set the stage for the first digital computers and all the electronic devices we use today. Thomas Edison’s curious observation opened the door to a new world of electronics that continues to evolve and change our lives.