Now that I've introduced you to resistivity, I wanted to talk to you about why different materials have different resistivity. The reason I find this so interesting is that resistivity varies between different materials more than almost any other property. Take a look at this table. It shows the resistivity for different types of solid materials. So as we can see here, some metals have a resistivity of approximately 10 to the minus 8 ohm meters. While other materials like rubbers and polymers, they can have a resistivity greater than 10 to the 15 ohm meters. This means in our typical room sitting in front of you right now, you would have two objects whose resistivity differs by 23 orders of magnitude. This is just amazing. Can you imagine just about any other property of a material that differed by 23 orders of magnitude? If I increased my weight by that much, I would weigh more than the entire earth. If I increased my height by 23 orders of magnitude, I would be larger than a galaxy. So how is that the solid objects can have such different resistances? So the most basic explanation is that it comes down to how easily charged carriers, usually the electrons, are able to move through the material. If you've studied some chemistry, you might know that there are three ways that solid can be bonded together. So they are metallic, covalent, and ionic. So those are the three ways that solids can be bonded together. In metals, there are many unbound electrons which are free to move throughout from metallic lattice, which means that the electrons can move around very easily so they have a very low resistivity. However, in covalent bonds, electrons are shared between two different atoms, and there's very little capacity for the electrons to move. So these materials usually have very high resistivity, things like plastics and rubbers. In ionic lattices, what happens is one atom gives up its electrons to a neighboring atom. So overall, it's neutral, but any individual atom is charged. However, the electrons are still localized on individual atoms. And this means that usually, ionic compounds are good insulators. However, there are a few exceptions to this. So the first exception I wanted to tell you about is a covalent compound, which actually is a good conductor, and that is graphite. So here we have two chemical structures of carbon. On the left we have diamonds, and on the right we have graphite. It so happens that both diamond and graphite are covalently bonded. However, the diamond every carbon atom is bonded to four other carbon atoms around it, and all the electrons are localized between two atoms. But in graphite, every carbon is only covalently bonded to three other carbons, which means that there is one free electron on every carbon, which exists in between the layers. It's a much weaker bond that holds the different layers together. And these electrons are free to move between the layers, actually making graphite a good conductor. Another exception is that ionic compounds actually become very good conductors when they dissolve in water. On the left hand side here, I have a picture from the molecular structure of pure water. This is pure water, water that doesn't contain any ions, any salt or anything. As we can see, it contains one oxygen atom with two hydrogen atoms. However, there are no charged ions or atoms in pure water. So pure water is actually not a very good conductor. However, if you take a salt and ionic compound like this sodium chloride lattice and dissolve it in the water, then all of a sudden, the water will contain charged ions which are free to move around. And that actually makes the water a much better conductor of electricity. So adding salt to water will make it a better conductor. This property has been used by a man called, who goes by the name of Tesla Down Under. He's created a website where he uses a Tesla coil to create thousands of volts. He exposed himself to thousands, even millions of volts, and protects himself using objects of different resistivity. If you'd like to find out more about how he does this, check out the website, tesladownunder.com. There's some pretty cool videos and some pretty cool images of what he's been able to create.