What it shows:
Why doesn't a bird sitting on a high-voltage wire get electrocuted? This demonstration addresses that question and serves as a model of the situation.
How it works:
The important concept conveyed is that there needs to be a voltage difference across a conducting medium for current to flow through the medium. In this situation the conducting medium is a bird sitting on a high-voltage wire. The voltage on the wire is the voltage of the whole length of wire with respect to the ground. Although the bird on the wire would also be at this voltage, there is no part of its body that isn't. Since there is no voltage difference across its body, there will be no current flowing through its body, and thus the bird does not get electrocuted. If the bird could straddle a neighboring wire at a different voltage—ZAP! But this doesn't happen because power lines are strung sufficiently far apart. However, if a power line is down (on or near the ground) and a bird (or person) touches it—ZAP! In this situation there is a voltage difference across the bird or person, namely the difference between the wire voltage and ground.
Rather than a 20,000 volt transmission line, the demonstration operates at 120 volts from any outlet. Nevertheless, the "transmission lines" are bare (uninsulated) copper wires with no current-limiting resistors or fuses in line, so use caution in this demonstration. Since we don't want to zap birds, a light bulb becomes a surrogate for the bird—if there is a voltage (difference) across the light bulb, it lights (which is equivalent to ZAP). Otherwise, the bulb does not light, which indicates a safe condition.
A separate neon bulb is wired across the two transmission lines to indicate to the audience that indeed there is a voltage present, and also serves as a reminder to the demonstrator that the power is on and caution is in order. The "bird lightbulb" has two insulated wires attached (with alligator-clip ends)—they represent the bird's feet. Touching both of the clip ends to the hot or live line does not light the bird bulb. Neither does touching the neutral line. But, touching one clip to the neutral line and the other to the hot line lights the bulb—ZAP. Likewise, touching the hot line and ground results in a ZAP (you need to make sure that the clip touching "ground" is pushed through the sand and actually touches the aluminum plate, which is electrically wired to true ground). Touching the neutral line and ground should not light the bulb. If it does, your ground (or other wiring) is faulty and ought to be repaired!
Setting it up:
Plug it in and check it out (for correct outlet wiring), then unplug the demonstration so no unsuspecting person gets zapped. The lecturer should plug it in when ready to do the demonstration (and unplug it when finished).
This is a good visual demonstration that really emphasizes the important concept that there must be a voltage across the conductor for current to flow through the conductor. If you insulate yourself from ground (like standing on a plastic milk crate—don't rely on rubber soled shoes), you too can touch the HOT wire with your fingers and not get zapped.