A melted candy bar led to the invention of one of today’s most-used kitchen appliances. Percy Spencer was working on a military radar device in the mid-1940s when he noticed that his chocolate snack had gotten soft. Intrigued, he experimented with irradiating some kernels of popcorn, which promptly burst. Spencer’s work led to the first microwave ovens, which cost only a little less than a new car. On this date in 1955, the first consumer models were introduced, but they still required installation and cost $1,200 — about $10,500 in today’s money. Countertop models came along in 1967 and quickly became popular. Now, more than nine out of 10 homes across the country have a microwave oven, and manufacturing microwave ovens and other cooking ranges is a $2-billion a year business. Profile America is in its 17th year as a public service of the U.S. Census Bureau.