the United States Coast Survey, and during the next three years assisted in the transcontinental longitudinal determinations. Finding it necessary to make many experiments in determining the velocity of telegraphic time signals over long circuits, he made a thorough study of electricity. In 1869 and again in 1872 he was in Europe and made all the observations in the third and final determination of the difference of longitude between Greenwich, Paris and Cambridge. Subsequent observations by European astronomers confirmed his work. During 1874—6, he was preparing the results of his transatlantic work for publication, and during this period became acquainted with Graham Bell. On April 5, 1878, he tendered his resignation, which was accepted with the greatest reluctance to date April 15. During the four years that had elapsed since his return from Europe he had devoted all his leisure to experimental physics. It is recorded that in carrying on these experiments
Fig. 32.
Other workers were also successful in serviceably utilizing the 'loose contact' or microphonic principle in the telephonic transmitter. In January, 1877, Emile Berliner devised his well-known transmitter, for which he filed a caveat on April 14. It was referred to in the Washington Critic, May 18, and on June 4, 1877, he filed an application based on his caveat. The patent was issued January 15, 1878. On April 27, 1877, Thomas A. Edison filed his application for a patent on a battery transmitter; while in December, 1877, Professor Hughes commenced his now famous microphonic experiments, which were followed by Hunnings's employment of carbon granules. One of the first of the Berliner transmitters is illustrated in Fig. 32
Referring to some of these experiments with carbon electrodes, Sir William Thomson (now Lord Kelvin) wrote:
