A Jacob's Ladder is one of the easiest fun displays to make using high voltages, as seen in old sci-fi & horror films. Powered by a 12,000 Volt AC transformer, it creates a 1-inch spark.
I built this quick project in early 2020 using the same supplies as my Tesla Coil.
To make things more exciting, I coated the two electrodes with different metal salts, creating a rainbow of colors as the spark traveled up.
The color here is from lithium chloride, with its unique bright red color.