Page:Popular Science Monthly Volume 33.djvu/104

94 across its broad end, made a successful receiver. With the small end made to fit the ear, and the diaphragm end only a few inches below the water, the sound of a hand-bell has been received nearly a mile distant. Colladon and Sturm used a somewhat similar receiver, and heard a heavy bell ten miles away.

It was necessary to devise a better form of receiving apparatus. The Bell receiver and the Blake transmitter will not work under water. The first success was obtained by a form of transmitter resembling the Ader.

With this Prof. Blake transmitted and received signals between boats half a mile apart on the Taunton River in 1883. The transmitter was weighted to float at different depths, but in all positions as regards the approaching sound-waves it received equally well. Up to half a mile the signals from an ordinary dinner-bell were distinctly heard. These experiments seemed to indicate that a transmitter dependent upon a variable contact might yet be made which would work with satisfaction. This line was consequently followed up, and apparatus was devised by which signals were transmitted between boats a mile distant off Stone Bridge, near Newport, R. I., in the same summer of 1883 through a rough sea and in a dense fog. Various forms of microphonic transmitters were constructed, and experiments on Long Island Sound and on the Wabash River at Terre Haute, Ind., were conducted as opportunity permitted. One form of transmitter which worked fairly well consists merely of a diaphragm having within itself the elements of a microphone. It is placed in simple voltaic circuit with a Bell receiver. This diaphragm is made of hard carbon in granules about the size of smallest shot. A paste is made of these with rubber cement, and this in a mold and die under heat and pressure becomes a hard, thin, elastic disk. This diaphragm takes up the sound vibrations quite well out of the water. The action is similar to that of a multiple contact transmitter. On the river, however, through a long distance these did not seem sufficiently satisfactory. This difference in action between a long and short distance led to the thought that, as the advancing front of the sound-wave is an arc, approaching in curvature nearer and nearer the tangent to its circle, a large diaphragm would receive more sonorous energy and thus probably prove more effective. This is the point to which the experiments have now been carried, and the next trials will be with a diaphragm eighteen inches square. In October, 1885, signals were transmitted and received one and a half mile on the Wabash River from a locomotive-bell around three or four windings of the river, so that the operators were out of each other's sight and the sound