I don't know the name of the actual device, but I can tell you what it was doing.
Well if you've studied electrolytes in class you'll know that they contain free ions. A typical characteristic of salts is to break down into ions when in solution with water. Anyway, this device is applying a voltage to the solution and measuring (I assume) the current through the solution. In this case, more lights equals a higher current. Gatorade's purpose is to provide salts to the body after profuse sweating. This is why the result was similar to salt water. Both solutions have a high concentration of salt. Tap water, while it does have dissolved minerals, does not have nearly as many as Gatorade or a prepared salt water solution would have.
Current is the flow of electrons through a circuit. In diagrams it is usually indicated with an arrow. That isn't exactly true. Current direction is the general trend the electrons follow, in reality they are moving in every possible direction. The more free ions you have in a solution the more electrons you can have flowing through that circuit.
I've seen similar devices hooked up to a generator bicycle. The faster you turn the wheels the faster the magnet hooked up to it spins. It's the same thing as the giant rotors in power plants. The faster the magnet is spinning and reversing the poles in the coil the more electrons are going to be moving. The measurement tool is fairly similar, just the method to generate a current flow is different. I've never peeked inside one though. I'd be interested to see what exact circuits are inside one of these.
Sorry I can't help with the name though. It is generally called an ammeter(assuming I'm not handicapped and it is measuring current). Ask Muffinmix for a specific name.
