The Microwave Oven — Invented When a Radar Engineer Melted a Chocolate Bar

This patent describes an oven that uses microwave radiation — radio waves at a frequency of around 2.45 GHz — to heat food from the inside out. The magnetron (originally a radar component) generates microwave energy that penetrates food and causes water molecules to vibrate rapidly. This molecular vibration generates heat throughout the food simultaneously, rather than conducting heat inward from a hot surface. The patent covers the arrangement of a magnetron power source, a waveguide to direct the microwaves into a cooking cavity, and a containment structure that reflects microwaves back into the food rather than letting them escape.

The clever bit

Spencer was working with radar magnetrons at Raytheon in 1945 when he noticed a chocolate bar in his pocket had melted while he stood near active radar equipment. He immediately recognized this as something new. His first deliberate experiment was with popcorn; his second — with an egg — resulted in an explosion when the egg's internal steam pressure built faster than the shell could release it. Spencer built a metal box to contain the microwave energy and filed a patent. The first commercial unit, the Radarange, weighed 750 pounds and cost $5,000 — it was used in restaurants and ocean liners. The countertop home microwave didn't arrive until the 1960s and didn't become common until Amana (a Raytheon subsidiary) released a $495 model in 1967.

What it does not cover

• ×The magnetron itself — the microwave-generating tube was invented separately for radar applications
• ×Inverter microwave technology — modern microwaves use variable-power inverters rather than cycling on/off; not covered in this original design
• ×Combination convection-microwave ovens — hybrid designs that add conventional heating elements
• ×Industrial microwave applications (food processing, materials curing) — these use the same principle at much higher power