hello this is Professor Keane and welcome back to my lectures on electricity magnetism and light in this series of lectures I'm talking about the experiments by Hans Christian ersted that he published in 1820 in which he describes his work on the effect of a current electricity on a magnetic compass needle in the last lecture I talked specifically about his experimental apparatus which consists of a galvanic cell a battery essentially attached to a wire and he ran an electrical current through this wire and noticed that it had an effect on a magnetic compass needle there's an apocryphal story that I'm well I'm not sure if it's true but it's a nice story anyhow so I'll tell it the idea is that that ersted was having a dinner party where he was showing off to his friends that if you pass a sufficiently large electrical current through a wire the wire turns red hot and glows this is what all of us scientists do at our dinner parties it sounds like a lot of fun and he noticed out of the corner of his eye that when he did so there was a compass needle nearby and there's a small turn or deflection in the compass needle and he noticed this but didn't make a big deal of it at the dinner party but later on he went back and started redoing the experiments and that is how he ended up making the discovery that he did anyhow we described this first experiment that he carried out in my last lecture and let me remind you of the apparatus consisted of a battery I'm going to draw schematically this battery like this I'm going to make it look a bit like a car battery this isn't exactly like the one he used his looked more like a bunch of trays of acid sitting on the floor with copper and zinc plates sticking out of the top he had this battery and then he had this wire that he called The uniting Wire and it ran along in the north-south direction so I'll put some Compass directions right here that would be the Eastward Direction West and Southward Direction and this was propped up in his laboratory on a couple of stands that looked a bit like this you know he doesn't have as I mentioned my last lecture he does not have any drawings in this text so we're I have to imagine what it looked like and then this was attached via these connecting wires like this and the positive terminal of the battery was here and the negative terminal was right there and then he noticed when he hung a compass needle beneath this or nearly the compass needle points in the Northerly Direction but when he attached the wire to the positive terminal what happened is this compass needle swung in this direction it swung oops I drew that in the wrong way so it swung in the westward Direction like this okay so that is what he noticed that's essentially the the experiment that he carried out well um let's talk about some of the variations this was the first experiment he carried out now it might the question might arise immediately is this deflection of the compass needle can it be understood in terms of simple coulomb attraction and repulsion I mean we already know that positive charges repel each other and negative charges repel each other and positive and negatives attract so maybe this is just some interesting manifestation of something we already knew by virtue of Coulomb's law so what does he do for his second experiment this is a variation on the first experiment for the second experiment what he does is uh instead of suspending this from a string he just kind of mounts it on top of a bracket of some sort that still allows the compass needle to turn but then he moves this wire to the East and to the West Above This compass needle so let me draw a top view of how I imagine this looking so if he takes this compass needle that's below this wire and so the wire that's the top view there's this is hooked up for the positive terminal of the battery this is a negative terminal of the battery so this bottom part is this end of the wire and the top is the right end of the wire so this is a Northerly Direction like that and if you slid this off to the right you know this compass needle is still pointed kind of in this direction and he says well um maybe this is because this wire overall is acquiring a positive charge so suppose the wire itself The uniting Wire is acquiring an overall positive charge okay so I'm going to draw a positive sign in the wire to indicate that this hypothesis acquiring a positive charge and maybe this compass needle is turning this way because the tip of the compass needle is positively charged and the back of the compass needle is negatively charged so when you try to run an electrical current through this with the battery it becomes positively charged and this is turning because it's repelling the positive charge on the tip of the compass needle maybe that's what's happening well if that is what was happening then if you moved the wire off to the left side that is you moved it more West in the Westerly Direction what should happen well then this compass needle As you move this wire to the left to the west then the company needle should swing back in the other direction because after all moving it left or right shouldn't change whether or not the wire has an overall positive charge so if we move the posit the presumably positively charged wire to the west of the needle of the Encompass needle the needle should reverse so all of the things being the same now the compass needle that had been pointing the tip had been pointing Westward it should now deflect to the east because after all we were hypothesizing that the tip of it was positive that the back was negative so we moved this thing off like this it should swing the other way but this is not what happens he notices that when you move this off to the left you're keeping it above it the needle is still pointing the same direction so this is not what happens and so he concludes from that that the coulomb repulsion coulomb repulsion is not an adequate explanation of the deflection of the I guess Westward deflection of the compass needle Beneath The Wire okay you can kind of see how he's thinking about this okay so there's something else going on here something stranger going on here so he continues let's talk about his third experiment okay uh where is this by the way if we look at the text this appears on the uh we are about at the top of page 78 bottom of page 77 and top of page 78. and in this section what he does is he replaces the wire with different kinds of material wires so he tries wires made of different materials okay so he mentions that he tries wires that were constructed of platinum he uses gold he uses silver he uses iron so platinum gold I'll use the chemical symbol silver he uses iron wires he uses some other aloes like brass and zinc I think and he notices that when he does so this you still get the same effect you still get or he's still got the same deflection of the compass needle although with some different wires the effect was less pronounced so but with in some cases somewhat less pronounced results or pronounced turning or has used called pronounced declination pronounced turning of the needle okay and he mentions that even if you break the the circuit this Loop right here by interposing a bit of white of water in a cup you still get the same effect so imagine you took this wire and you took once again this battery and you had these terminals on the battery and you hook the wire up like this and this end of the wire that's at the positive terminal of the battery you dip it into a cup of water okay so let me get my color here so you put some water in this cup like this so now you've got water in the cup and the other wire you dip in the same water and you attach it well in this case you know you still get the Turning of this compass needle but it only um it only works if you have these wires quite close to each other in the water so he says I think if you only have them separated by just a couple of inches then you still get the Turning of this compass needle um if you have it wires too far apart then you do not get the Turning of the compass needle okay and and presumably what's happening is these different materials uh in some way resist the amount of electrical current that can flow through the wire so you might imagine if this is a positive terminal negative terminal you're going to get some electrical current I'll call it I this electrical current through the wire and when you use different materials that have less conductivity then it diminishes the amount of electrical current through the wire and that reduces the the size of the effect he's talking about but as long as you have electrical current that's sufficiently large you still can get a deflection of the compass needle I think what I'll do is in my next lecture talk a little bit more about the relationship between the electric potential that the battery produces the voltage the battery produces the amount of electrical current through the wire and the resistance of the material so I'll do a little bit of a digression into that next time and then we'll come back and talk more about some more of ersted's experiments