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)
- 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
- 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.
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!