The Vacuum Tube may be the greatest discovery and invention by mankind. Discovering that electrons can flow from a heated element to a plate in a vacuum, then discovering that by placing a fine metal screen across the path of this electron flow with the voltage of the screen hindering the flow to the plate, the concept of amplifying a weak signal was thought up. Until a better way to regulate current for amplification came by way of the transistor, vacuum tubes were used to create everything electronic, from radios and televisions to computers. Transistors can do it with lower power requirements and a lot less space, but every circuit we use was invented using the tube.

Using a heated element to cause electron flow to a metal plate is called thermionic emission. Because electrons flow from the heated element to the plate, the flow is strictly one way. This means that if AC (See Ohm’s Law) came to the element, it would only allow the part where the voltage was negative with respect to ground to pass through. All positive voltage would block electron flow to the plate. This one way tube became known as a diode. Diodes quickly became useful for converting AC to DC by using two or four diodes to keep the voltage positive or negative throughout the AC flow (called rectification). It was also discovered that a diode can act as a detector to separate the audio from an amplitude-modulated (AM) radio signal. Still, radios required headphones in order to hear the audio.

The Audion Triode was invented by Lee de Forest in 1906. It had a heating element called a filament, a plate, and a grid – the Control Grid. Using Ohm’s Law (See Ohm’s Law), we know that there is a voltage drop across a resistor depending on the amount of current flowing through the resistor (V = IR). Connecting a resistor between the plate and a high voltage source, the current change will change the voltage at the plate. Zero current will cause the plate voltage to be the same as the source voltage and the higher the current, the lower the voltage at the plate. Any radio signal traveling into grid will change the current flow at a higher rate than the signal’s flow and the voltage swing will be much higher at the plate than at the grid. Thus, the signal at the plate will look like a bigger version of the signal coming into the grid. The signal’s voltage will be inverted (upside down), but the signal phase is not usually an issue.

Using the filament was not real practical, so a cathode was placed between the filament and the grid and as long as the voltage is negative with respect to the plate voltage, electron flow isn’t hindered. Filaments became heated through a separate power source. The Audion was not a perfect vacuum and the residual gas had a blue glow. After de Forest sold the patent to AT&T, it was discovered that better vacuums created higher electron flow from the filament. Making better tubes created problems. The space between the grid and the plate (also called an anode) acted as a capacitor and thus would convert the tube into an oscillator (a circuit that generates waveforms). A special plate transformer was created to eliminate the oscillating, but it was discovered that placing another grid between the control grid and the anode, called a screen grid, solved the problem with capacitance. The inventor of this tube, called a Tetrode, was Walter H. Schottky, who would later be important in transistor development.

Now, another problem arose. The screen grid could hinder the plate current at higher voltages, so another grid, the suppressor grid was placed between the screen grid and the anode and the Pentode was invented. All of this by 1926, along with the early invented cathode-ray tube, meant that not only radios could be improved, but other ideas became reality. Tubes could also convert light to electrons and convert the electrons back to light. Television was first experimented by the time the pentode tube was created.

Not only could the tube be used for signal amplification, but also for switches. Unlike mechanical relays, vacuum tubes (and transistors) were fast. Although tube failures could be an issue, keeping filaments on constantly minimized failures. Thus computers like the ENIAC and the first UNIVAC were built with tubes.

Eventually, the transistor replaced the tube. With special exceptions, transistors are better. They require no glass, no heat and no vacuum. They’re now made so small, millions can take up an area the size of a fingertip. Tubes needed to be spaced as to not make too much heat. A computer with 20,000 tubes is quite big and the computing power of those matched that of the first solid-state calculators. A smartphone is more powerful than computers that used to take up large rooms by a wide factor.

This is just a simple explanation of how tubes affected our lives. Nikola Tesla may have discovered how to transmit signals through the air, but without the vacuum tube we would not have radio or television like it is today. No such discoveries are taking place today; instead we are just moving these things around in a box and calling it something different. The inventors would be shocked to see what we can do today, but they also would be shocked to see what we can’t do. Just watch those futuristic movies from ninety years ago. Their advancement of technology was far greater than today. We do not have the ideas, the imagination, or the wisdom they had back then. The invention of the vacuum tube is why technology is where it is today.