Millikan Experiment

in uh 199 or thereabout Robert milikin did some experiments to show that charge comes in natural units following on some earlier evidence he was showing that there are grains of charge and the grains are all the same size I don't mean the same volume or the same mass the same size in charge electrically as measured by electrical forces on them if uh for example we have one grain of charge it's not a DOT of course in a given electrical situation then there will be a certain force on it if we have another identical grain the same Force if there are two grains and let's say a small object here in the same electrical situation the force will be twice as big with three grains on an object it would be three times as big one 2 three with this kind of evidence we can tell whether charge comes in natural units in this film Dr Alfred Redfield is going to do experiments similar to milans using this apparatus on these thick metal plates he will put charges from a battery to connected to these wires you'll see the battery in the switch later on plus charge on one plate minus on the other sometimes the other way around that will put electric forces in here between the plates on any small charged object any little grain of charge that we put between these plates we'll have to put a very tiny object to carry our grains of charge in there because we want to measure the force on the smallest amount of charge that ever occurs in the world we have to use objects so small that a microscope is necessary to see them we get the particle in between the plates here look at it through the microscope and see it in light Sent in from the side this microscope will do in the film we'll use a somewhat better microscope to be sure to see clearly in order to get the little particles carrying the charge into this space between the plates we're going to use some tiny plastic spheres shake a few into the air above this glass tube and let a few drift down through the air through the tube through a hole in the bottom of the tube into the space here between the plates the tube has a hole in the bottom as you see so when they drift down the tube a few will pass through the hole into that space the bottom of the tube is made of metal when it's in place here it completes this upper metal plate comes out flush with it across here so that we can get a well-defined electrical situation with charge on the top plate charge on the bottom plate exerting forces on those few grains of charge on a little plastic sphere that we've dropped down through the air and through the hole into that space now here's red field with the apparatus over on the right is the microscope and the camera in the back bit to the left is the source of light shining in between the plates there they are the thick metal plates connected Through the Wire on the left to the batteries those four batteries down there and with this knob Redfield can put in anything from one to four of them with this switch red field can connect the batteries to the plates when the switch is in the up position one end of the battery is connected to one plate the other to the other when it's in the down position it's just reversed and in the middle position the battery is cut out and the plates are connected together so there's no electrical force on a particle between the plates then now red Field's putting a wire in through the hole into the space between the plates so that he can see how his microscope focuses on it he's almost got it in Focus now there it is Now by using this ruler Redfield can see how big the field of view through the microscope is those marks are a millimeter apart now Redfield is going to put some particles in the space between the plates he takes them out of this water suspension on a toothbrush particles of oil different sizes so we'll see how they behave sprays them in the air over the glass cylinder there and some of them will drift down through the cylinder through the hole into the space between the plates here they come falling down moving at steady velocity pulled down by gravity now Redfield put some charge on the plates and some of those particles are charged you can see they're pulled up by the electric force we've put the knob of the switch box on the screen over at the left so you can see which way the plates are charged now if we take the charge off everything falls turn it around some fall and some go up but they're the other ones back and forth and you'll see that there two sets of particles charged oppositely so when one goes up the others pushed down and when the first one is shoved down the second's pulled up in all these shots you'll notice that each particle moves at its own steady speed let's take another look at the particles as they came in driven downward by gravity alone when the particles are driven down by gravity they quickly speed up and then as they speed up the drag of the air rapidly increases until it exerts a force equal and opposite to the gravitational force downward when those forces are equal the particles move constantly this whole process of speeding up takes place so rapidly that you probably never noticed the speed up process even when Redfield turned on and off the electric field but you did notice that the speeds were constant and they were different for the different particles all those differences arise because the particles were all different kinds sizes masses as well probably charges we can get rid of most of those differences and get a standard Behavior by using particles of the standard size and mass when we have particles which are all the same any differences in behavior that are left over arise solely because there are different numbers of grains of charge on the particles so we're going to take standard particles standard plastic spheres 1.8 microns in diameter that's 1.8 * 10- 6 M and 2.8 * 10us 14 Newtons in weight these are made for electron microscopy here's a picture of a few taken through an electron microscope here come some of the little plastic spheres now falling down driven by gravity notice that they come down at the same speed now Redfield is about to try to balance them by putting the right amount of charge on the plates got a little too much so they're going up now he decreased the number of batteries you just about got them in Balance although there are two standard spheres visible here Redfield will keep focused on the one on the right because Brownie and motion will eventually separate them now we'll want to be able to keep them in balance and know what balance is so Redfield is marking position of the knob so that we can always know exactly where we are that position is almost exactly three batteries with three batteries on the plates the sphere is in Balance the amount of charge on the plates will call the set charge whenever we have the set charge we'll know the sphere with this amount of charge on it would be in balance between the plates by cutting the charge off the plates removing the electric force we can let the sphere fall there it goes down by putting on the set charge we can stop it it's in Balance all over the field of view of the microscope to keep working with it we'll have to be able to pull it up so Redfield is adding some extra batteries normally these are not in the circuit when the lever the switch box is pushed to the upper position here the extra batteries are there more charges on the plates and the sphere is pulled up no charge on the plates it falls set charge it's in Balance extra charge more Force up it goes with this technique we can keep the particle in the field of view of the microscope while we do experiments the experiments like the Milan experiment that we're eventually going to do in which we change the charge on the sphere let's consider for a moment how we're going to measure the added charge or the charge subtracted when we do change the charge on this sphere right now the sphere is in balance that means that the electric force upward is just equal although in the opposite direction from the El the gravitational force downward like this electric force and gravitational force now when we change the charge on the little sphere here there will be an additional electric force that's the force that's going to drive the little sphere along suppose we add some charge and the electric force gets bigger it gets bigger by an amount proportional to the charge and that added Force will'll call the drive force in this case remember it's the added electrical force here it is up this way now under the influence of that Force the sphere will speed up and come to a steady velocity when the drag of the air is again in equilibrium pulling in the other direction that steady velocity to which the sphere comes is a measure of this added electric driving force we know for spheres of this kind that the force driving them along results in a speed of motion just proportional to itself we know this but the first thing that Redfield is going to do is to check up on it he can check up on it by allowing the sphere to fall first freely no electric force in the region the sphere is then driven by the gravitational force alone and he can measure its speed now to get a second known Force so we can test the proportionality of the speed to the force he can use this electric force the one that keeps the sphere in Balance can do it this way he's already got the gravitational force he can't turn that off on the sphere and now he takes the electric force that bounces the sphere and turns it upside down and adds it on top of the gravitational force that is the same thing as the balancing the balancing electric force he turns that upside down You Remember by using the switch box and taking the positive lead from the battery and the negative lead and reversing them so the whole Electric System is turned upside down the electric force now pushes down and that way he has first one force and then exactly double the force we can now see that the speed under the one force is exactly half of the speed under two forces here the sphere is in Balance redfield's going to measure how fast it falls under Gravity he'll need a grid and a watch in order to measure the speed across the grid the grid there is the same as the one he can see the watch will make 1 second intervals and we'll put 1 second ticks on the soundtrack for you to hear now he'll drop it for five seconds what about 23 on the right hand scale across that Grid in 5 seconds Redfield do it again so we can get an accurate measurement of the Velocity 24 just about the same now in order to see that the the force is proportional to the speed we're going to need a double Force to see whether we get the double speed Redfield will put that on after he holds the sphere up by turning the electric force the balance force upside down 48 with the double force in 5 seconds almost exactly double you can see already that the speed and the force are proportional once again make it 48 again so the speed doubles when the force doubles we'll need that knowledge because we are going to keep on using the speed as a measure of the force now that we know that the two are proportional now we're ready for the main experiment with this x-ray tube redfield's going to change the charge on the little sphere putting a shield around the tube to protect himself with an opening so that the x-rays can go right in between the plate when they change the charge on the sphere will have added charge on it added force on it driving it away from balance from the speed with which it moves we can find out the charge change here the sphere is in Balance now he's going to add x-rays try to change the charge it's moving he must have changed the charge on the little sphere and P it up with the same charge on the plates the set charge with which it used to be in Balance now he's dropping it down so that he can measure the added charge by the speed with which it'll come up 12 divisions in the same 5 Seconds enough charge was added on there so with the extra electric force on it same set charge on the pipes it came up 12 divisions in 5 seconds now I'll try to change it once more you added x-ray probably changed the charge so he's dropping the sphere down so we can measure what the charge is yes you got a change in charge 22 divisions in 5 seconds this time a bigger charge on the sphere tried changing it again and I think he succeeded yes it seems to go up faster the amount of charge since Bounce has been increased I think and we'll measure it by hauling It Up From The Bottom once more with the set charge on the plates 46 a lot more charge on it that time try changing it again not sure whether he succeeded yes it seems to go slower I think this time he knocked charge off and after he's dropped it down we'll be able to tell how much charge there's left on there charge difference since balance 34 he did get some off a minute he'll try changing it again going pretty slow I think he knocked more charge or of course it was still going up so that there must be some added charge since balance 13 you did get a lot of charge off now he was going to try changing it again I think he didn't quite succeed though it seemed to speed up and slow down again may it put some on taking it back off again by having the X-ray too long about 11 I think it's really the same amount of charge on there just a different measurement of it have to try changing it once more it started to fall there did you notice I think he knocked quite a bit off and he could only hold it up up and haul it up with the extra charge so this time he'll have to drop it down because he's got charge off since the balance we'll be able to see how much came off 12 and downwards so it got off enough so that the added electric force on that opposite sign of charge pushed it down at 12 so Redfield can go on changing the charge on the little sphere with x-rays measuring what it is with a force on it until at sometime he loses it now here he still has it but in that change you see even with the extra charge he can't hold it anymore it's falling down set charge extra charge no matter what he does reverse field doesn't help it's just gravity's pulling it down there because there's no more charge on the little spere that ends his run and there are all his numbers everything he measured if you look at them you see they're no random set of numbers they fall in groups groups around 11 and 12 bound double it 22 23 triple it round 34 so let's take a guess let's guess that these are all just about whole number multiples of let's say 11.8 now we'll draw a graph and plot all this data in ascending order of size and compare it to spacings 11.8 Apart 1 * 2 * 3 * 11 8 over on the left side you see the downward one a little bit over the 11.8 then to the right of it a point in Balance once he got the same charge that he had to start out with to the right of it a group around the plus 11.8 the 11.8 upward the next group about double it triple it and so on so you see that the charge that came on and off the little sphere always came on and off in natural units one of these units two 4 three and so on never anything measured by this speed somewhere in between 11.8 and Z never anything but whole number multiples of a natural unit of charge now what about the charge that was on the little sphere when it was in balance is that also a multiple of our fundamental unit of charge to see that look at this measurement the measurement of the speed of fall of the drop under gravity gravity is balanced as you see here by our electrical force with just a set charge so the speed of fall on gravity is a measurement of that electrical force and a measurement of the charge on there so these measurements measuring the charge through the speed of fall at 23 or 24 show that there are just two natural units of charge on the sphere just two of the natural units when we started out with the sphere in balance from these experiments we should take away two kinds of knowledge some specific numerical facts and a general conclusion let's go to the numerical facts first when we put charge on these plates which are almost exactly 3.1 mm apart we were able to balance one of our standard spheres between the plates with two Elementary charges on it we put the charge on these plates from these batteries with the knob here which selected out three batteries in fact we could just as well use three of these batteries connected in series then connected to the plates then we can actually find out that these three batteries do put one of the Spheres with two Elementary charges on it in Balance the sphere weighs 2.8 * 10- 14th nton so the electrical force up must be that big on two Elementary charges the electrical force from three batteries putting the charges on the plate is therefore just 1.4 * 10- 14th nton on any Elementary charge between the plates these numerical results will be useful to us later but it's far more important to look at the general conclusion from this whole set of experiments in which we put charge on took charge off shooting x-rays the little sphere what we found out was that there are Elementary units of charge but nature provides grains of charge all the same size for that we always saw one or two or three of these grains or some other whole number through the forces that's the really important conclusion the existance of a natural unit of charge in which all charges come