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Most of the many possible types of microphones as they are now known to exist were tried during the few years immediately following Bell's original invention. The more important types are described, dated, and identified as to their inventor. During the last 15 years, the theoretical and experimental technique for the test, analysis and development of the microphone has advanced greatly with the result that many of the early types have again become of interest. The development of the vacuum tube amplifier during this period, moreover, has removed the old requirement of large electric output from the microphone and greatly altered the practical aspects of the situation. The refinement of the microphone during these 15 years is sketched and the characteristics of some of the more important recent instruments are given.

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