A Jacob's Ladder is a basic spark-gap device. As described in Wikipedia...
"A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms, ionizing the gas and drastically reducing its electrical resistance. An electric current then flows until the path of ionized gas is broken or the current reduces below a minimum value called the 'holding current'. This usually happens when the voltage drops, but in some cases occurs when the heated gas rises, stretching out and then breaking the filament of ionized gas. Usually the action of ionizing the gas is violent and disruptive, often leading to sound (ranging from a snap for a spark plug to thunder for a lightning discharge), light and heat.
Spark gaps were used historically in early electrical equipment, such as spark gap radio transmitters, electrostatic machines, and x-ray machines. Their most widespread use today is in spark plugs to ignite the fuel in internal combustion engines, but they are also used in lightning arrestors and other devices to protect electrical equipment from high voltage transients."
You can easily make one for that "Dr. Frankenstein's Lab" look. They're great for trick or treat. To make the one in this photo, I took a boiler transformer, the old magnetic type, not the newer electronic ones. I removed the electrodes and replaced them with #12AWG solid copper wire. The wire was shaped so that at the closest point, they were separated by approximately 1/4". As the wires rose up, the separation became greater. Plug it in and you're off!
A note of caution, I did this in my garage so as to not accidentally catch anything on fire. Also so curious pets or kids don't stick their noses into the arc. (Think taser.)