DIY Cathode Ray Tube: Interacting With Electrons




Electrons: they are all around us, but we cannot really see or interact with them. Luckily, there is a device that will let you. A cathode ray tube, or CRT, is a glass tube with electrodes on either end. When there is a vacuum drawn and a high voltage across the two electrodes, a cathode ray forms. This is a stream of electrons that looks like a brilliant blue-purple line of plasma. You can manipulate and bend this stream of electrons with a magnet. This line not only looks cool, but it can be used to prove a scientific theory; electrons are a subatomic unit with a negative charge. I'll explain how it proves this theory later in this instructable. This piece of test equipment that I will show you how to build can be used as an awesome scientific demonstration for any physics and chemistry class, or it can be used just as a way to confirm a long standing scientific principal.

The video below compliments this instructable with a visual demonstration of this Cathode Ray Tube in action.

Lets Get Started!

Step 1: How a CRT Works

CRT's have been around for a long time. In fact, the first one was built in 1897 by scientist Ferdinand Braun. These tubes have been used as television screens for all of the 20th century and for the early part of the 21st century. They are still used widely today as oscilloscope screens, but these cathode ray tubes as screens are a little bit more complicated than the simple cathode ray tube that we are going to be building in this instructable. So many of you may be wondering now, how does a cathode ray tube work?

A cathode ray tube consists of a ray going from the cathode to the anode, and it is made up of electrons. The simplest cathode ray tube is a sealed glass chamber that has electrodes at either end and a port to be able to draw a vacuum from. If you draw a vacuum on the chamber and apply a high voltage to both ends, then a blueish purple glowing line forms. This is the cathode ray. Now the reason this ray forms is because of the high voltage. The cathode, because it has a negative high voltage with respect to the anode, will have a large amount of electrons on it. These electrons want to flow to the anode to equalize the voltage, but cannot because there are too many air molecules blocking their way. If you remove most of the air in the chamber though, then the electrons are able to flow from one electrode to another. Because the vacuum is not perfect, there will still be air molecules in the tube. When the electrons in the ray hit an oxygen atom, it moves to a higher energy state. Because it cannot stay at this high energy state for long, the atom releases this energy as light. This is why the beam has a blueish purple glow.

The reason that you can bend this ray with a magnet is because electrons are negative. This allows them to be manipulated by a magnetic field. It was this property of the cathode ray that let JJ Thompson discover and prove the existence of the electron as a subatomic unit of an atom in 1897. You can read more about his experiment at this website:

These cathode rays are able to be used in televisions because they can be effected by magnetic fields. A TV CRT has an electron gun in the back that shoots a beam of electrons at a phosphor screen. The tube then has two electromagnets that are able to change the electron beam's direction in the X and Y axis'. If you apply two different electronic signals to the electromagnets and varying power to the electron gun, you can create an image on the screen. This is like a paintbrush painting hundreds of horizontal lines of with varying levels of blackness on a canvas.

Well, now that you know how a CRT works and is used today, lets build one!

Step 2: Materials

For this project, you will need a few materials and tools.

For tools, you will need:

  • A Bunsen burner (You can use a butane soldering iron, a commercial burner, or you can build your own)
  • Pliers
  • A screwdriver
  • Wire cutters/ strippers
  • Hot Glue Gun

For materials, you will need:

  • Glass test tubes
  • Copper Wire
  • Nails/screws(For the electrodes)
  • Vinyl aquarium tubing
  • alligator clips
  • hot glue
  • refrigerator compressor
  • copper wire
  • wood
  • rubber sheeting
  • Mechanical pencil

You will also need a high voltage power supply. This can be almost any power supply that can produce a high voltage at a relatively high current. For this project, I used my ZVS Power supply.

You can read my instructable on how to build it here

Or watch my video on it here:

Step 3: The Vacuum Setup

The vacuum setup is what removes the air from the test tube. My vacuum setup is a wooden vacuum chamber that I built for other experiments, but it can be used for this one too. It functions using a refrigerator compressor connected via a vinyl tube to a wooden platform made of particle board. The tube seals with the copper pipe on compressor and seals to the wooden platform with the tip of a mechanical pencil. The glass chamber of the CRT seals to the wooden base with the vacuum hole with a piece of rubber sheeting with a hole in it. To learn more about this vacuum chamber setup, watch this video:

The vacuum setup is essential to making your CRT work. Without it, there will be too much air for the electrons to freely travel. Now, to making the actual CRT!

Step 4: Blowing a Hole in the Glass

The first step to making a CRT creating a hole for the anode. To do this, first found a glass test-tube. I then set the tube at the end of the fire from my homemade Bunsen burner. This heats the glass to the point of it being pliable. After it is heated, use a pointed object to make an indentation on the tip of the test tube. After this, reheat the test tube on the flame again. Once it is hot, blow into the open end of the tube. This will make a bubble that will eventually pop, creating a hole in the top of the test tube.

Step 5: Fixing the Hole

After you have your hole created, it will be jagged from the popped bubble. to fix this and make it less dangerous, use a butane torch, or your Bunsen burner, to melt the jagged glass and make it more smooth.

Step 6: Adding the Anode

This is probably one of the hardest steps. To add the anode, you will need to take a nail or a screw, and insert it into the hole you made in the previous step. You will need to make sure that the point of the screw is inside the tube and there is a good amount of it hanging out of the tube to connect to your power supply. You then need to heat the glass again, and while holding the nail in the hole, use pliers to crimp the glass around the nail. This step takes a little while, but with patience and a steady hand, it can be done. After you are done with this step, the nail should stay put inside the tube without wiggling or falling out.

Step 7: Hermetically Sealing the Tube

Now, in the previous step, the sealing of the hole will not hermetically seal the tube. By this I mean that despite making sure that the screw does not move at all in the glass, air can still escape. This will make the CRT not work because the air will always get back in. To seal it, you can just use hot glue. To do this, use a hot glue gun to deposit glue around the area where the screw meets the glass. This should seal your tube.

Step 8: Setting Up the Vacuum Chamber and Cathode

The cathode of the CRT will be routed through the vinyl tube from the inside of the CRT to the metal of the fridge compressor vacuum pump. To do this, you will first need to take take a long length of bare copper wire, and push it though the tubing from the compressor end. Stop when you have about an inch of copper wire sticking out of the vacuum port on the CRT end of the vinyl tubing. On the other end, cut the wire to about 3/4 inch and bend it into an arc. Finally, insert the copper wire into the copper pipe of the fridge compressor and the vinyl tubing over the copper pipe. you can now use the metal body of the compressor as the ground, or cathode, of the CRT.

Step 9: Vacuuming Down the CRT

After your tube is made and the cathode/vacuuming rig is set up, turn on the compressor by plugging it in. Then, take a piece of rubber sheeting and cut a small hole in the center. After this, set the rubber on top of the hole and set the CRT on top of the rubber. The glass CRT will suction down to the rubber and air will start being removed. It will take about 2-4 minutes to remove enough air to create a cathode ray.

Step 10: Connecting the Power

While the CRT is being vacuum down, you can connect the ZVS high voltage power supply. This power supply was home built and can provide over 35000 volts at about 10 mA. You will need to use alligator clip wires to connect the cathode, or ground of the power supply to the cathode of the cathode ray tube. This is the metal of the compressor. After the ground is connected, connect the anode(top) of the tube to the anode(red wire) of the power supply. Now, your Cathode Ray Tube setup should be functional.

Step 11: Fire It Up!!

To turn on your CRT, wait until it has a full vacuum, then plug it into a Variac transformer. Turn on the variac, then turn up the voltage until you see a purple-blue glowing line appear in your tube! Now, it is time to play!

Step 12: It Works! Electrons Exist

Your cathode ray tube should now be working. The glow looks fantastic, like some otherworldly streetlight. You can then interact with ray using a hard drive magnet. Just hold the magnet near the tube, and the beam will deflect to either move toward the magnet, or away from it. You are actually able to touch electrons though a magnet! Something you were not able to previously touch is now touchable. It is fun to play with this cathode ray tube with a magnet. You are actually proving that electrons exist and are negatively charged by moving these magnets. Because magnetic fields affect charged particles, the fact that a magnet can effect the ray shows that whatever makes up the rays are negatively charged. In fact, this cathode ray tube is a reenactment of the experiment by J.J. Thompson that proved the existence of the electron as a subatomic particle.

This experiment is fun, but do not run it too long. This tube is dissipating about 100 watts, so without the proper cooling, it will melt the seal and burn anyone who touches it. It also uses lethal high voltage, so DO NOT TOUCH THE ANODE!

Overall, a Cathode Ray Tube is an awesome apparatus to build and really fun to play with. It is also cool because is proves scientific principals.

Thanks for reading and make sure to vote for me in the Untouchable Contest, Power Supply Contest, and Scientific Exploration Contest!

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    24 Discussions


    2 years ago

    CRTs are so cool! Maybe I'll build one.

    Any particle of the atom could be a deflect, reflect and or interact in the matter even in presence of a magnetic field... All of the particles including the electric force of the atoms could change from angle, neutralization and polarization when taking attention external factors can be achieved any physical chemical manipulation... Also I did on 1983 some kind of experiment and let me tell you that no matter wherever it goes the deflection ray, always to find where the exit of the particles caused by a Flyback... Great experience I had when I realized that experiment to prove the movement of the electrons, but protons also acts because they are part of the electrical charge and nobody for sure know how does really the electricity moves through any state or matter... I recommend you have very careful using this way of demonstration taking great care the amount of Voltage and Current using this method of CRT... : D

    1 reply

    2 years ago

    WAAAY COOOOOOL!!!!! Explaining how one works was very helpful and IF I can build several, will look great in my Halloween Laboratory- you got my vote!

    2 replies

    Reply 2 years ago

    I wouldn't recommend using it as a Halloween decoration. It could be pretty dangerous if you are too exposed to the X-rays.


    2 years ago

    Just bending the beam with a magnet does not prove the electrons are negative. Positively charged particles will also deflect in a magnetic field (though the other way).

    Also, be careful with HV.


    2 years ago

    Hi, did you test this for RX generation? High voltage in this way usually generates X ... watch out.


    2 years ago

    Most of the refrigerator compressors (piston types) I have tried will not produce a vacuum better than 1 mm of mercury.

    The best I could get was by drilling a hole in the side of the cylinder, just above the piston at it's lowest point, using that hole as the vacuum line, and using a second pump as a second stage.

    I could never use a mechanical pump to get a vacuum good enough to produce X-rays. Leaks, oil vapors, and out-gassing were always a problem.


    2 years ago

    Please correct me if I'm wrong, but I think this tube may be a good source of X-rays, very dangerous! I wouldn't run it for long for that reason alone.

    If you lower the pressure of the tube more (but still not a vacuum) you can see other phenomena, like Crookes dark space, and other kinds of banding. (I made a similar kind of tube back in the '80's using pennies for the electrodes) Something else really interesting I observed was teeny-tiny ball lightening that drifted down the tube.

    Have fun with your tube but be safe!

    2 replies
    Kalle Klæpthrobscottle

    Reply 2 years ago

    I've made something similar like you did, also back in the 80's when I was a kid. X-rays (Röentgen rays), will occur when one places an angled Tungsten plate as an anode and a coil/filament as a cathode.

    I wouldn't be too concerned about X-rays... However, his contraption with open electrics, are more of a safety matter. A very serious one, indeed..!


    Reply 2 years ago

    There would be some low energy X-rays coming out of it, much as with any high voltage spark gap or a neon tube. But the pressure is much too high to make it a dangerous source of such radiation. It's primarily a discharge tube, the 'beam' is visible as gas ions relaxing from an excited state, not an electron beam.

    Kalle Klæp

    2 years ago

    You seem like a very bright kid. You also explained the circuit very well. However, you realize that you handle a high voltage device with an output current more than capable to kill you..?

    You didn't even bother to put at least a plastic cover or similar over the device, whilst its turned on..! The on/off switch is dangerously close to the transformer that itself has a potential to kill you instantly. Sooner or later, the missing safety actions will be the cause of serious injury or even death...which is "Not good" to use your own words in the video.

    Anyone trying to play around with this stuff....DO NEVER EVER have an unshielded high voltage device plugged in..! It's an accident waiting to happen and there is no way back once you electrocuted yourself..!


    2 years ago

    Nice experiment but I think you have actually made a nitrogen discharge tube. I don't think the vacuum from a fridge compressor is anywhere near low enough for an electron tube and without any gas in there I think you would need to heat the cathode to get electrons to flow and have a phosphor screen to be able to see them.

    The purple glow is from nitrogen plasma.

    1 reply

    Reply 2 years ago

    Nophead is correct. It is mostly nitrogen ions giving off visible light as they return to a lower energy state. Though there would also be some oxygen, argon, CO2, etc - everything we have in the atmosphere. A spectroscope could be used to detect emission lines from the different gases in the tube. There are also electrons in the tube, but their movement is not directed as they would be in a CRT.


    2 years ago

    This Instructable DOES have useful instructions for home
    experimenters wishing to make a high voltage discharge tube. It DOES NOT
    result in a "Cathode Ray Tube", for multiple reasons described below.
    Please do not call it that! It is simply not correct terminology for
    what this is, and you will only confuse people getting started in this

    First, the common flyback-type power supply used here
    actually generates a high-frequency pulsed AC output, essentially a
    Tesla coil with a solid state oscillator on the primary. There is no
    rectification, smoothing or reference to ground at the high voltage
    side. There is no "anode" or "cathode" terminal, so in reality the
    discharge is flowing in both directions between the electrodes - exactly
    like a neon sign or fluorescent light does. You would need a high
    voltage diode and capacitor after the secondary winding to get a DC
    output, which is a fundamental requirement for generating Cathode rays.

    the refrigerator vacuum pump is good for experimenting with
    low-pressure gas discharge tubes, but they are far from being able to
    generate the high vacuum necessary to generate electron beams. The good
    news is, this also makes this tube safer to be near, as it is not an
    efficient X-ray generator. The bad news is, the high-voltage discharge
    inside the tube is randomly shaped, like a lightning bolt. The discharge
    is principally using gas ions (mostly nitrogen and oxygen nuclei) going
    through excited states, not electrons! It is possible to influence the
    ion discharge with an external magnet using a tube like this, but not to
    control it in a repeatable way.

    Third, an essential part of the
    Cathode Ray tube concept is to have a fluorescent screen at the cathode
    end to visualize the impact of the electrons flowing toward it. Without a
    fluorescent screen, it would be a Crookes tube (assuming the first and
    second conditions above were met). Which is a predecessor to the CRT.

    normally a separate heating filament would be used to reduce the
    breakdown voltage required to reach a distance between anode and cathode
    - this was standard in vacuum tubes and fluorescent lights - making
    them a "cold cathode" tube (sort of confusing, as these are usually AC driven as well, so both electrodes can be the cathode).

    It's great that you want to share your enthusiasm, and it's a good starting point. But please reconsider the name of this project - it is absolutely NOT a CRT or a CRT demonstrator. It is a high voltage discharge tube, or a Geissler tube if you want to be fancy about it. :-)


    2 years ago

    You scared the beans out of me with that first video... trusting your life to the insulating quality of a 99 cent screwdriver.

    On the plus side, I think you did quite a good job explaining how a ZVS works. And, seeing that awesome buzzing arc makes me want to build one for myself!

    3 replies

    Reply 2 years ago

    This high voltage power supply is not the same as the ignition coil in your car that puts the very low amperage spark for your spark plugs. My ZVS power supply puts out amperages that are over the maximum letter amperage. It will kill.


    Reply 2 years ago

    You said: "This high voltage power supply is not the same as the ignition coil in your car .... My ZVS power ... will kill."

    Umm... that was kinda my point.


    Reply 2 years ago

    High voltages such as this at low amps is generally non fatal.. It is almost the same as grabbing a spark plug wire on a running engine.. I have never heard of anyone being killed by a spark plug jolt..