On to the next vacuum tube: The Hull Magnetron. Magnetrons happen to be the last vacuum tube type to be widely sold to consumers - every microwave oven has a beefy magnetron inside. However, these are cavity magnetrons, which will be discussed another day. Today I'll be discussing Hull Magnetron vacuum tubes. This device allows for crude transmission of radio/microwave radiation using a single vacuum tube. Its development led directly to the more advanced cavity magnetron in use today.

It's important to know how the basic vacuum tube rectifier works before continuing. You can either read my post on it here, or read the brief overview I wrote for the triode post, copied below:

A metal filament and a metal terminal are separated by a vacuum gap. A battery heats up the filament until it glows, causing electrons to be ejected from the filament via a process called thermionic emission. Applying the positive terminal of a battery to the metal terminal and the negative terminal to the filament causes current to flow across the tube, bridging the circuit, via free electron charges. Reversing the polarity causes no current to flow, as electrons are attracted back into the positive filament. In this way, the simple vacuum tube rectifier acts as a diode and only allows current to flow in one direction.

Using Magnetism to replicate the Triode

The triode is a basic improvement to the rectifier tube that adds a conductive mesh between the filament and anode. This essentially lets the tube act as a switch that can reduce, increase, or completely stop the flow of current across a tube using electrical signals. This lets it work as an oscillator and amplifier, and makes it very similar to certain solid-state transistors. Note that the triode uses only the electric field of the mesh, filament and anode to control the flow of charges (electrons).

The Hull magnetron is a type of vacuum tube that uses a magnetic field to control current flow and, incidentally, create radio/microwaves. They aren't used much (if at all) today but are an easy-to-understand, pretty cool vacuum tube example and a stepping stone to the cavity magnetron in your microwave oven.

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Suppose we build a vacuum tube rectifier using two cylinders. An inner thin cylinder contains a heater and a filament, and is connected to the negative end of a battery. A larger hollow cylinder surrounds the inner cylinder, serving as the anode, and is connected to the positive end of a battery. If this assembly is placed in a vacuum and the filament is heated up, it will act as a vacuum tube rectifier, like a diode, just as before.

But now, if this new tube is placed in a magnetic field, things change. Say that the magnetic field runs parallel to the tube: The field lines line up with the cylinders. This can be approximated by winding a coil of wire around the tube and running current through it, or placing two ring/dipole magnets above and below the tube. Now the electrons emitted from the central cylinder are influenced by a radial electric field and a magnetic field parallel to the cylinders.

Remember that when a charge moves through a magnetic field, it experiences a force known as the Lorentz Force:

F = q(v x B)
Where v is the charge's velocity vector and B is the local magnetic field vector.

What this means is that the electrons no longer follow a straight path between the filament and the anode, and the tube no longer acts like a simple rectifier. The above equation shows that the path of the electron will now be curved as it accelerates in the electric and magnetic field. If we use a very strong magnetic field, the electrons don't even reach the anode: They curve back into the filament! This has the effect of shutting off the current across the tube, since no free electrons ever cross the tube. If we use a relatively weak magnetic field, the electrons will follow curved trajectories but still reach the anode and we still have a tube that effectively acts like a rectifier. In this way, we now effectively have a triode that works using a magnetic field instead of an additional electric field.

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Hull Magnetrons produce microwaves

You might ask at what point the electron stops reaching the anode and curves back into the filament. This occurs at a specific magnetic field strength called the Hull cut-off field. When the magnetic field applied equals the Hull cut-off field, the electrons emitted from the filament are barely able to reach the anode. Any stronger field, and they will curve back in, cutting off the flow of current. It's around this field value that the device behaves most like a triode.

An interesting side effect of this device occurs at the cut-off field. Some of the electrons slightly miss the anode, and fly past in a circular trajectory. The electrons in this trajectory will eventually hit something or otherwise leave the trajectory somewhat quickly, but the fact that this occurs at all has useful consequences.

When charges accelerate, they radiate electromagnetic radiation. This is how braking radiation works in XRay tubes: High speed/energy electrons impact a metal, and when one of these electrons hits a nucleus head-on, they decelerate very quickly and produce XRay electromagnetic radiation. The circulating electrons in the Hull magnetron are also accelerating - but their speed doesn't change significantly. Travelling in a circular trajectory requires a constant downward acceleration - think satellite orbits, or swinging a bucket over your head. This acceleration is provided by the Lorentz magnetic force. The electron doesn't gain any energy (magnetic fields can't do work) but it does change direction.

This constant acceleration of the electrons trapped in circular electromagnetic orbits within the tube thus radiates energy away in the form of electromagnetic waves. These are not XRays this time - they are much lower frequency radio or microwaves. Many different frequencies are emitted here, but a peak occurs at the cyclotron frequency of the electron - basically its frequency travelling around the circle.

This means that just by building a different shaped rectifier tube and adding a magnetic field, we can produce electromagnetic waves. In fact, magnetrons allow for high power, high frequency emissions, up to dozens of GHz (for perspective, many cheap Ham radio transmitters are somewhere around 0.2 - 0.4 GHz, Wifi and microwave ovens are 2.4 GHz, and high frequency Wifi is 5 GHz).

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This special tube then allows for a relatively simple way to make very high frequency EM waves without a complicated circuit. To my knowledge, Hull magnetrons aren't really used in the present time, but more complicated magnetrons called cavity magnetrons are extremely common ... particularly in your kitchen.

The Hull magnetron is unfortunately unstable and not very efficient. But it does represent one of the first ways to make radio- and micro- waves using a single vacuum tube, and one that operates on a pretty simple combination of static electric and magnetic fields at that.

It seems to me like making a Hull magnetron using a rectifier tube could be a do-able DIY experiment. I may try this when I get the chance.

I hope you learned something from this post. Let me know if anything is unclear or incorrect.

Thanks for reading!

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