How to Make a Barometer with a Jar & Balloons

Hi I'm Gary for Expert Village. The next thing we want to do is prepare the balloon to cover the jar. And what we need to do is cut the balloon towards the bottom and we're going to then spread it across the top of the jar so we're going to make a cut right down pretty much into the middle of the balloon. Now this balloon should be a twelve inch balloon, they usually say on the packages the size and inches of the balloon that you can blow it up to. And it's very important, most balloons are nine inches and this should be a twelve inch balloon. It's a little bit bigger and it'll give you a more flexibility to put it over the jar, the mouth of the jar. And it should be as flat as possible over the mouth of the jar. So we go ahead and spread it out and cover the jar with it and we have to make sure we have some a little bit of room on the sides so that we can we're going to seal it by wrapping a string around. So we pull it really tight around the mouth of the jar and try and get it as flat as possible. But be very careful not to rip the balloon and this basically puts a really tight seal over the jar. And what we're trying to do is to create an air tight jar and measure the air pressure from the outside as it sits on the balloon. We're going to measure the weight of that air pressure as it sits on the surface of the balloon and we're going to be able to tell which is heavier if the air pressure in the jar becomes heavier than the balloon from the bottom will be rising up and that will affect our gauge or if the air pressure from the outside is stronger it will press down on the balloon and that will also affect our gauge in a different way. The gauge being the straw that's going to be sitting on the surface of the balloon. So that's how we put the spread out the balloon we cut the balloon and make sure that it is as flat as possible and that we have a very nice air tight seal over our jar.