546 



HISTORY OF SCIENCE. 



sounding-board g. A is a hollow box with a wide trumpet-mouthed 

 opening in one side, and at the top is a circular opening, over which 

 is stretched a membrane u. A very small disc of platinum m attached 

 to the centre of the membrane is caused by its vibrations to complete 

 the electrical contact by which the current from the battery B flows 

 through the circuit including the coil C. When a person sings into 

 the opening at A the membrane is thrown into vibration, and at each 

 vibration a click is emitted by the sounding-box at //. Such was the 

 apparatus contrived in 1860 by M. Reiss of Friedrichsdorf, who thus 

 successfully solved the problem of causing electricity to reproduce 

 sounds at a distance. Other forms of musical telephones were sub- 

 sequently proposed; but in 1876 Mr. GRAHAM BELL, who had for 

 some years devoted himself to the study, had produced the speaking 

 telephone, which Sir W. Thompson has not hesitated to call the wonder 

 of wonders. By this instrument persons separated by hundreds of 



i: 



FIG. 260. 



miles may converse, and even recognize the tones of each other's 

 voice. Here no battery is required, for the vibration of a thin iron 

 plate is made to generate the currents. Fig. 260 is the apparatus in 

 section. H s is a cylindrical steel magnet, about 3 inches long. One 

 end is enclosed by a small coil, B, of fine silk-covered copper wire, the 

 extremities of which pass through the wooden case M at //, and are 

 connected at i with the line wire c c. By the influence of the magnet 

 the iron diaphragm L L is made magnetic, and when it is set vibrating 

 by speaking into the opening R R', its approach to and recession from 

 s act according to principles which will be mentioned in the following 

 chapter, to induce momentary currents in the coil and through the 

 circuit. These currents are received by an exactly similar instrument, 

 and by the inverse actions the iron diaphragm of the latter is thrown 

 into vibrations which reproduce those of the transmitter. 



The other invention, the phonograph of Mr. EDISON, dates from 

 1877. In the phonograph a membrane thrown into vibration by speech 

 or other sound slightly presses a blunt steel point against a sheet of 

 tinfoil drawn along at a uniform rate. If the membrane were quiescent, 

 the point would mark a very shallow trace on the tinfoil, which is sup- 



