How did Ben Franklin discover that lightning was electrical energy?

Ok, I know this is bad... I should look this up or something... I get that he tied a key to the top of the kite. Metal conducts electricity, but string does not. So how did he prove that lighting was just the product of electrical energy. I would think that he would have to connect it to a circuit to do some work in order to prove that it is electricity. The mere fact that lightning struck the key doesn't prove anything. Please shine some insight on this conundrum. Thanks!