 If you take a glass tube, put 2 bits of metal in either end, vacuum out all of the air, and then apply a high voltage current across it, then what you'll end up with is called a cathode ray tube. And this device has been used in all sorts of different applications, including the original television sets and computer monitor displays. So what is this ray made out of? Well, it's made out of cathode ray particles, but what are cathode ray particles, and how do we know anything about this ray at all? Well, there's a couple of observations we can start to make about cathode rays. The first one is that the ray can be deflected by electric fields. This observation tells us that the ray has a charge, since it experiences the electric force. Secondly, the ray casts a shadow if an object is placed in its way, so we can work out the direction of the movement of the particles as they go through the tube. And lastly, we know the charges must be negatively charged, since they feel an electric force in the opposite direction of the electric field, and because they obey the third left hand rule, which only applies to negative nature of charged particles. Now if we start to manipulate those electric and magnetic fields, we can do some interesting things. First of all, let's pick a particular electric field value. Maybe we'll have the electric field point down, and that'll create an electric force pointing upwards. Then we'll come along with a magnetic field that is perpendicular to both the electric field and the direction the particles are moving. If we direct that magnetic field such that the magnetic force will be in the opposite direction of the electric force, then we can adjust the magnetic field strength until those two forces are exactly balanced. And when the particle feels balanced forces, it moves with an undeflected or uniform motion. Now let's do some math. We can write a balanced forces statement showing that right now the electric force equals the magnetic force. And since we know the equation for electric force is charge times the electric field strength, and for magnetic force is charge times the speed times the magnetic field strength, we can substitute those equations in as well. Canceling of the charge and doing some algebra shows us that we can determine the speed of the particle, v, from the magnitude of the electric field, and the magnetic field needed to create uniform motion. So we can determine the speed of these particles, but how does that help us figure out what they are? Well, if we turn off the electric field now and just allow the particle to move through the magnetic field, then the particle is going to move in a circle, because particles moving in magnetic fields have uniform circular motion. And we can measure the radius of that circle. As the magnetic force is creating centripetal motion, I can write another balanced forces statement, this time making the magnetic force equal to the centripetal force. Substituting in the equation for magnetic force from before, and mv squared over r for this centripetal force, and cancelling out one speed variable on either side, gives us this expression which is sort of useful? Because the problem here is that we can't solve this equation if we don't know what the charge of the particle is, and we don't know what the mass of the particle is. But physicists are clever, and they realize that you can solve for what the charge divided by the mass was. You can just bring all of that to one side of the equation, and what you'd have left over is called the charge to mass ratio. The charge to mass ratio of an unknown particle can be found by dividing the speed of the particle by the product of the magnetic field and the radius of the particle's curvature. So why is this charge to mass ratio even important? Well remember we're trying to figure out the identity of this cathode ray particle, and we can send lots of other particles through the same apparatus to do some process of elimination. Like we can try a hydrogen ion, or an alpha particle, and we can figure out the charge to mass ratio of these particles to see if they're cathode ray particles. In 1897, physicist JJ Thompson performed the charge to mass ratio on cathode ray particles, and he made a few important discoveries after doing so. First of all, he found that the charge to mass ratio for these particles was unique. It wasn't the same as any other particle that had been measured before that. Secondly, the charge to mass ratio for cathode rays was huge, and that meant one of two things. Either these particles had a really, really large charge, or an incredibly tiny mass. And we know today that the latter is actually true. And lastly, if Thompson changed the metal used in the cathode ray out for a different type of metal, he got the exact same beam with the exact same charge to mass ratio. And this was really important. It means that whatever these particles are, they're inside of elements, inside of lots of different types of metals. Which means they must have something to do with the very foundation of atoms. And I think it's amazing that a pretty simple device paired up with some of the physics that we're learning in Physics 30 has led humans to understand what the inside of atoms look like, and led us to discover one of the elementary particles in the universe, the electron.