The invention: System of high-speed cooking that uses microwave radition to agitate liquid molecules to raise temperatures by friction. The people behind the invention: Percy L. Spencer (1894-1970), an American engineer Heinrich Hertz (1857-1894), a German physicist James Clerk Maxwell (1831-1879), a Scottish physicist The Nature of Microwaves Microwaves are electromagnetic waves, as are radio waves, X rays, and visible light. Water waves
and sound waves are waveshaped disturbances of particles in the media—water in the case of water waves and air or water in the case of sound waves—through which they travel. Electromagnetic waves, however, are wavelike variations of intensity in electric and magnetic fields. Electromagnetic waves were first studied in 1864 by James Clerk Maxwell, who explained mathematically their behavior and velocity. Electromagnetic waves are described in terms of their “wavelength” and “frequency.” The wavelength is the length of one cycle, which is the distance from the highest point of one wave to the highest point of the next wave, and the frequency is the number of cycles that occur in one second. Frequency is measured in units called “hertz,” named for the German physicist Heinrich Hertz. The frequencies of microwaves run from 300 to 3,000 megahertz (1 megahertz equals 1 million hertz, or 1 million cycles per second), corresponding to wavelengths of 100 to 10 centimeters. Microwaves travel in the same way that light waves do; they are reflected by metallic objects, absorbed by some materials, and transmitted by other materials. When food is subjected to microwaves, it heats up because the microwaves make the water molecules in foods (water is the most common compound in foods) vibrate. Water is a “dipole molecule,” which means that it contains both positive and negative charges. When the food is subjected to microwaves, the dipole water molecules try to align themselves with the alternating electromagnetic field of the microwaves. This causes the water molecules to collide with one another and with other molecules in the food. Consequently, heat is produced as a result of friction. Development of the Microwave Oven Percy L. Spencer apparently discovered the principle of microwave cooking while he was experimenting with a radar device at the Raytheon Company. A candy bar in his pocket melted after being exposed to microwaves. After realizing what had happened, Spencer made the first microwave oven from a milk can and applied for two patents, “Method of Treating Foodstuffs” and “Means for Treating Foodstuffs,” on October 8, 1945, giving birth to microwaveoven technology. Spencer wrote that his invention “relates to the treatment of foodstuffs and, more particularly, to the cooking thereof through the use of electromagnetic energy.” Though the use of electromagnetic energy for heating was recognized at that time, the frequencies that were used were lower than 50 megahertz. Spencer discovered that heating at such low frequencies takes a long time. He eliminated the time disadvantage by using shorter wavelengths in the microwave region. Wavelengths of 10 centimeters or shorter were comparable to the average dimensions of foods. When these wavelengths were used, the heat that was generated became intense, the energy that was required was minimal, and the process became efficient enough to be exploited commercially. Although Spencer’s patents refer to the cooking of foods with microwave energy, neither deals directly with a microwave oven. The actual basis for a microwave oven may be patents filed by other researchers at Raytheon. Apatent by Karl Stiefel in 1949 may be the forerunner of the microwave oven, and in 1950, Fritz Gross received a patent entitled “Cooking Apparatus,” which specifically describes an oven that is very similar to modern microwave ovens. Perhaps the first mention of a commercial microwave oven was made in the November, 1946, issue of Electronics magazine. This article described the newly developed Radarange as a device that could bake biscuits in 29 seconds, cook hamburgers in 35 seconds,and grill a hot dog in 8 to 10 seconds. Another article that appeared a month later mentioned a unit that had been developed specifically for airline use. The frequency used in this oven was 3,000 megahertz. Within a year, a practical model 13 inches wide, 14 inches deep, and 15 inches high appeared, and several new models were operating in and around Boston. In June, 1947, Electronics magazine reported the installation of a Radarange in a restaurant, signaling the commercial use of microwave cooking. It was reported that this method more than tripled the speed of service. The Radarange became an important addition to a number of restaurants, and in 1948, Bernard Proctor and Samuel Goldblith used it for the first time to conduct research into microwave cooking. In the United States, the radio frequencies that can be used for heating are allocated by the Federal Communications Commission (FCC). The two most popular frequencies for microwave cooking are 915 and 2,450 megahertz, and the 2,450 frequency is used in home microwave ovens. It is interesting that patents filed by Spencer in 1947 mention a frequency on the order of 2,450 megahertz. This fact is another example of Spencer’s vision in the development of microwave cooking principles. The Raytheon Company concentrated on using 2,450 megahertz, and in 1955, the first domestic microwave oven was introduced. It was not until the late 1960’s, however, that the price of the microwave oven decreased sufficiently for the device to become popular. The first patent describing a microwave heating system being used in conjunction with a conveyor was issued to Spencer in 1952. Later, based on this development, continuous industrial applications of microwaves were developed. Impact Initially, microwaves were viewed as simply an efficient means of rapidly converting electric energy to heat. Since that time, however, they have become an integral part of many applications. Because of the pioneering efforts of Percy L. Spencer, microwave applications in the food industry for cooking and for other processing operations have flourished. In the early 1970’s, there were eleven microwave oven companies worldwide, two of which specialized in food processing operations, but the growth of the microwave oven industry has paralleled the growth in the radio and television industries. In 1984, microwave ovens accounted for more shipments than had ever been achieved by any appliance—9.1 million units. By 1989, more than 75 percent of the homes in the United States had microwave ovens, and in the 1990’s, microwavable foods were among the fastest-growing products in the food industry. Microwave energy facilitates reductions in operating costs and required energy, higher-quality and more reliable products, and positive environmental effects. To some degree, the use of industrial microwave energy remains in its infancy.Newand improved applications of microwaves will continue to appear.