up the American Appliance Company on July 7, 1922, to build silent refrigerators, with Bush and Smith among its five directors, but changed course and renamed it the Raytheon Company, to make and market the S-tube. The venture made Bush wealthy, and Raytheon ultimately became a large electronics company and defense contractor. Starting in 1927, Bush constructed a differential analyzer, an analog computer that could solve differential equations with as many as 18 independent variables. This invention arose from previous work performed by Herbert R. Stewart, one of Bush's master's students, who at Bush's suggestion created the integraph, a device for solving first-order differential equations, in 1925. Another student, Harold Hazen, proposed extending the device to handle second-order differential equations. Bush immediately realized the potential of such an invention, for these were much more difficult to solve, but also quite common in physics. Under Bush's supervision, Hazen was able to construct the differential analyzer, a table-like array of shafts and pens that mechanically simulated and plotted the desired equation. Unlike earlier designs that were purely mechanical, the differential analyzer had both electrical and mechanical components. Among the engineers who made use of the differential analyzer was General Electric's Edith Clarke, who used it to solve problems relating to electric power transmission. For developing the differential analyzer, Bush was awarded the Franklin Institute's Louis E. Levy Medal in 1928. Bush taught boolean algebra, circuit theory, and operational calculus according to the methods of Oliver Heaviside while Samuel Wesley Stratton was President of MIT. When Harold Jeffreys in Cambridge, England, offered his mathematical treatment in Operational Methods in Mathematical Physics (1927), Bush responded with his seminal textbook Operational Circuit Analysis (1929) for instructing electrical engineering students. In the preface he wrote: Parry Moon and Stratton were acknowledged, as was M.S. Vallarta who "wrote the first set of class notes which I used." An offshoot of the work at MIT was the beginning of digital circuit design theory by one of Bush's graduate students, Claude Shannon. Working on the analytical engine, Shannon described the application of Boolean algebra to electronic circuits in his landmark master's thesis, A Symbolic Analysis of Relay and Switching Circuits. In 1935, Bush was approached by OP-20-G, which was searching for an electronic device to aid in codebreaking. Bush was paid a $10,000 fee to design the Rapid Analytical Machine (RAM). The project went over budget and was not