Microwave History

Birth of the microwave oven

The Story Of Raytheon, RADAR & The Microwave Oven

The first practical microwave ovens were made by the Raytheon company. The Raytheon story starts with Charles G Smith, who wanted to make refrigerators, but did not have enough money to start a company.

In 1922, Mr Smith built a working refrigerator which was good enough to persuade his friend, Laurence K Marshall, and Marshall's friend, Vannevar Bush, to invest in his ideas, and they brought other investors' money to the table. Bush was a lecturer at MIT (which has a bigger part to play later), Marshall was an engineer, businessman, and close friend of Bush.

Having raised the capital to start The American Appliance Company, they worked for three years to produce a commercially viable refrigerator, and failed. They recognised that their product could not compete in the marketplace, and they had used up most of their starting capital.
Somewhere in the development process, Marshall and Bush had asked Smith what alternative uses there were for the specialised pumps the company had developed to make the refrigerators, and Smith had pointed out that they might be used to make thermionic valves (aka vacuum tubes in the US). Now, faced with the failure of the company, Marshall and Bush urged Smith to concentrate on making vacuum tubes. So, in 1925, Smith came up with a practical design for a diode, the "gaseous rectifier" (it had a trace of noble gas in the vacuum, which improved the design over previous rectifiers).

At the time, all radio sets operated from batteries, designated by letters A, B, C, H. All radios needed batteries A and B (some needed three or four) and the most expensive was the B battery, which supplied the high voltage HT (high tension) supply for the valves. Think of this as being like 80 to 100 AA pen cells connected together, and you get an idea of the size needed to give the 120V to 150V anode voltage for the valve. These were not cheap, either!

The "gaseous rectifier" was an instant success - the radio sets of the US were now able to work from the AC mains. The valve revolutionised the American radio industry, marketed as the Raytheon rectifier, and thousands were sold. The company was a success, but now they had to change the company name, because another firm had a prior claim to be called The American Appliance Company. In 1927 the company was renamed Raytheon, after its biggest selling product.

In 1925, when production of the rectifiers started, the company employed a remarkable man named Percy L Spencer. He had an insatiable urge to understand things. From a poor background, he started work in a paper mill at age 12 years. At 16, he learned to be an electrician by joining a small team carrying out the electrification of the mill. Fascinated by radio, he joined the US navy to learn the skills of telegraphy, and while "on watch" was able to study textbooks, from which he came to understand the workings of the wireless telegraphy apparatus. Whatever knowledge he needed to understand things, he learned for himself from books. His amazing ability to learn "on the job" allowed him to suggest improvements to almost all the production processes, with positive results beneficial to the company. Despite his lack of formal qualifications, he rapidly rose to the post of Chief Engineer. It was later said of him that he could make any valve you wanted from "a milk bottle, a sardine can, some fence wire and a bucket of whitewash".

The concept of RADAR (radio detection and ranging) stemmed from work begun in the early 1900s, with significant contributions from scientists across the world up to the start of World War II, when it became apparent that a working RADAR system would give great advantage in aerial warfare.

Simply put, RADAR works by transmitting a narrow focused band radio waves or microwaves, and receiving reflections of the transmitted waves. The waves are reflected by objects in their path. The time delay between the transmitted pulse and receiving the reflected signal can be used to find the distance to the reflecting object. The basis of a RADAR transmitter at that time was effectively limited to two devices which could produced focused bursts of electromagnetic energy, the magnetron and the klystron.

The Varian brothers, working at Stamford university, had published a description of the klystron valve in 1939, and the Germans based their RADAR systems on the device - but their klystrons were low power devices, transmitting only about 10W . The klystron radiation frequency was much more consistent than the only competing device, the cavity magnetron, invented by Hans Hollmann and patented in 1935 (Berlin). Randall and Boot, of the University of Birmingham, developed an improved magnetron. The magnetron was able to produce transmissions of much greater power, but the output frequency drifted in use. However, Randall and Boot showed that it was possible to synchronise the frequency of the transmitted energy to the receiver, and so give a practical RADAR device with about 100W transmitting power. Cooperation on the project with the GEC research laboratory soon resulted in the production of magnetrons with 6kW output power! However, it required extremely careful work to manufacture a cavity magnetron with suitable characteristics for the purpose. It took a skilled engineer over a week to make one magnetron core, and, with a high rejection rate, these were not cheap devices!

In 1940 the UK government sent Lord Tizard to the US on a mission, accompanied by several other "military scientists", to meet key US industrial and academic organisations with the objective of carrying forward secret projects that the UK had neither resources nor time to develop. Magnetron production was one of these projects, and "No.12" magnetron was sent on the journey. Ignoring the other projects -

A contract was given to the Bell Laboratories division of Western Electric to produce magnetrons, but the US Government had recently set up a Radiation Laboratory at MIT as part of the National Defense Program instigated by Franklin D Roosevelt. In order to facilitate discussion, the MIT Radiation Laboratory had asked Raytheon, their local vacuum tube manufacturer in Cambridge, Mass., to contribute, and Percy Spencer was delegated to attend the Friday meeting with the Tizard Mission. Although he was only officially attending the meeting as an observer, Percy was able to display enough knowledge to convince the UK delegation that it was a good idea to lend him the No.12 magnetron over the weekend - this precious valve which had been escorted onto the ship at Liverpool by a platoon of guards!

When Monday came around, Percy was able to make suggestions for potential manufacturing improvements which convinced the UK mission that they should award Raytheon a small contract for magnetron production. The Bell contract was for over 200/week, the Raytheon contract was for 17/week - but of course, demand increased as the war went on, and Percy Spencer found ways to make them "better and cheaper".
By the end of the war, due to the improved manufacturing methods introduced by Percy, Raytheon were making 80% of the world supply of magnetrons. It is probable that they were also the only concern making magnetrons at a profit!

As an aside, it should be mentioned that the UK "production magnetrons", made by GEC, improved the design by adding an extra cavity. Since all the US production was based on the specification of the prototype which was shown to them by the Tizard Mission, US made magnetrons always have one cavity less than UK made magnetrons. It should also be mentioned that the MIT Radiation laboratory designed more working RADAR installations than any other facility during WW II - over 100 different systems.

So, by 1944, Raytheon had become a major defence contractor. During testing of a RADAR system, a candy bar in Percy's pocket was partially melted - and it gave him an idea. He tried using the magnetron array to make popcorn - and it worked!

Now, it was common knowledge that the microwaves emanating from the working magnetrons could warm things up, but no-one had previously tried to use magnetrons for cooking. Percy worked hard at the idea of a microwave oven, and had an oven working well enough to warrant a patent application in October 1945.


In 1947 Raytheon were marketing the Radar Range microwave ovens to commercial outlets that needed to heat food quickly - they were expensive ($2k-$3k), big and very heavy, and they needed an industrial electricity supply, so they were out of reach for domestic users - but they were the first microwave ovens to be built.

In the early 1950s, domestic appliance makers became interested in microwave ovens. Having no infrastructure to sell to the public, Raytheon licensed their knowhow to the Tappan Stove Company in 1952. In 1955, Tappan sold the first domestic microwave oven, but priced at $1300 they did not sell well.

In 1965, after Percy had retired, Raytheon bought Amana, a maker of deluxe refrigerators, with the intention of using Amana as a vehicle to produce a domestic microwave. They succeeded - using cheap Japanese magnetrons that cost about $25 each, as opposed to the $300 that small Raytheon magnetrons sold for. Those Amana microwave ovens were on the market in 1967 for $600. They were still at luxury prices but much more affordable, and, as time has told, the microwave oven is now a commonplace kitchen item.


Links:
You want to repair your microwave? In Europe, there is a legal obligation to check that the shielding is working after a repair. Don't want to cook the family!
General repair information is available here - but remember, there are very high voltages in a microwave, be sure you know what you are doing.

Readers Digest feature - Percy L Spencer - his grandson's website.

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