A melted candy bar in an engineer's pocket helped lead to which kitchen appliance?

Answer

Raytheon engineer Percy Spencer noticed it near a radar magnetron in 1945. Retellings differ on which candy it was.

Context

Percy Spencer was a self-taught engineer at Raytheon who worked on radar equipment during World War II. A magnetron is the vacuum tube that generates microwaves for radar. According to the account in popular histories, around 1945 Spencer noticed that a snack in his pocket had melted while he stood near an active magnetron. He then tested popcorn kernels and an egg near the tube, and found they cooked or burst, which suggested that microwaves could heat food.

Raytheon filed a patent in 1945 and in 1947 introduced the Radarange, a refrigerator-sized and very expensive unit meant for restaurants and ships; it stood about 1.8 meters tall and weighed hundreds of kilograms. Home models arrived only in the 1960s and 1970s, with the countertop Amana Radarange in 1967 bringing the technology to households. Prices fell as the magnetron shrank and mass production improved.

Microwaves heat food because the oscillating electromagnetic field, usually at 2.45 gigahertz, makes polar molecules like water rotate and jostle, producing heat. The candy detail is the least reliable part of the tale: Spencer's own recollections and later retellings vary, naming a chocolate bar, a peanut cluster bar, or other snacks, which is why the entry notes that retellings differ.