Microwave Metal

Short answer: A microwave oven's magnetron fills the metal cooking cavity with microwaves at about 2,450 MHz, with a wavelength of about 12 centimeters. Food absorbs them, mostly through its water, fats and sugars.

Metal reflects them instead, and a conductor in the cavity acts like an antenna: the oscillating field induces currents and voltages in it. The field is strongest at sharp points, edges, corners and narrow gaps. There it can become strong enough to ionize the surrounding air, which then conducts and glows as a spark or arc, jumping to the oven wall or to another piece of metal. MIT materials scientist Caroline Ross compares it to lightning. That's why a pile of nails or screws, crumpled foil, metal twist ties or gold-trimmed china spark so readily. Smooth, rounded metal is less prone to arcing but not risk-free. The USDA says small amounts of new, smooth foil can be used to shield food, if the manufacturer's manual allows it, the foil is shaped smoothly to the food, and it stays at least an inch from the oven walls. It also says to remove it at once if arcing starts. Arcing can damage the oven and start a fire. The footage shows a heap of nails inside a microwave, sparking and glowing as the plate turns.

In collections:Everyday Physics