Proving air pressure is inversely proportional to volume

In engineering , we learnt that air pressure is inversely proportional to volume. I.e. p = 1/v Where p= pressure V= volume Provided temperature is constant. This means if pressure on material becomes less, its volume becomes more. Means it becomes bigger. If pressure is more then it becomes smaller. Now we will look at the experiment. All we need is a bell jar and partly blown balloon. Now keep the partly blown balloon inside the bell jar. Now take out the air which is present inside the bell jar using a vacuum pump. Now a surprising thing happens. The balloon which was inside the bell jar slowly starts to EXPAND !!!!!! No..... No...... This is not the work of a GHOST ! :-) It is pure science. Now we will look at what happened. ....... ........ ........ We have taken out all the air which was present inside the bell jar. Now balloon which is having little air, experiences pressure difference. In other words, the air outside the balloon is nil. So outside of balloon no air pressure. But balloon is having little air inside. So it is having little air pressure. As I said earlier, this difference in air pressure results in the increase in volume of balloon. The law stated the same ( see the start of article ) Increase in volume of balloon means it gets bigger by itself. How much it will go bigger depends on the difference in pressure. It is not the work of GHOST !!!!