Electricity & Magnetism

Argon Candle

In this exhibit, a power source pumps an alternating electric current into an electric rod. The current changes direction 13 million times per second. That means the rod goes from being positively charged to being negatively charged 13 million times per second, creating a powerful alternating field. The gas in the chamber is made up of charged particles called protons and electrons. Most of the protons and electrons are bound together by electrical attraction, but there are some free electrons in the gas. The free electrons are alternately attracted and repelled by the charged rod. They speed through the gas, colliding with other electrons and knocking them free of protons. When the free electrons fall back into orbit around the protons, they release energy that you see as glowing light. A pressure gage indicates the amount of gas inside the tube. The lower the pressure reading, the less gas in the chamber. When the chamber is filled with gas, the atoms are closely packed, and the electrons cannot travel far before colliding with each other. But when the tube is emptied, the electrons can get enough speed to bump each other free, and give off energy. Every gas gives off its own characteristic color when it releases energy. Air glows orange and purple. But when the chamber is filled with more argon than air, the glow turns whitish. As more argon enters the tube, a feathery white streamer rises from the top of the rod. The streamer, which is hot gas rising from the rod, disappears when the gas pressure in the chamber is too low. At low pressure, the hot gas spreads out too quickly to form a streamer. You can notice that the glow seems to follow your hand when the side of the tube is touched. The alternating electric fields are stronger between your hand and the rod than elsewhere in the tube. The electrons release more energy in the stronger electric fields, and therefore, glow more.