314 FORMS OF ENERGY cost $400 each, while, twelve years later, when 2,000,000 refrigerators were manufactured, the price was only $163. In 1914 a 60-watt electric-light lamp cost 43 cents, while a greatly superior lamp cost only 10 cents in 1941. Electric fans that cost $12.20 in 1914 could be replaced by superior fans at $2.75 in 1938. The first electric clocks sold for $30; by the time 2,500,000 a year were being sold, the price dropped to $4.00. One of the Most Significant Steps in Making Mass Production Possible Was the Development of Interchangeable Parts. Eli Whitney, inventor of the cotton gin, was largely responsible for the evolution of the principle of interchangeable parts. In 1818 he Fig. 105. Precision grinding one of the main parts of an electric locomotive. Both the part and the abrasive wheel, which is under the hood to the left, are rotated, the abrasive wheel at much higher speed. (Courtesy of the Westinghouse Electric and Manufacturing Company.) instituted the production of interchangeable parts for firearms. America has made such progress in the development of interchangeable parts that this principle is known in Europe as the American system.
MACHINES HAVE RAISED LIVING-STANDARDS 315 Industrial Accidents Have Declined. As the result of the development of a vast number of safety devices and other improvements industrial accidents have been reduced 61 per cent in frequency and 61.3 per cent in disabling injuries. Highly mechanized industries have the best safety records. Machine Tools Have Played an Important Part in the Advance of Technology. Watchmaker's precision, precision to a ten-thousandth of an inch, has been made possible by modern machine tools, which represent a major contribution of engineering, metallurgy, and invention to modern civilization. Machine tools are the reproductive members of the machine world. Machine tools are the machines that make machines. Fig. 106. A milling machine simultaneously finishing both sides of a large Diesel engine connecting rod. The two cutters have multiple teeth, similar to a circular saw. One of them is seen at the small end of the rod; the other is just behind it. The connecting rod is passed back and forth between the cutters which are rotated at high speed and moved closer and closer together with each pass until the rod is reduced to the desired thickness. (Courtesy of the National Machine Tool Builders' Association.)