Page:Popular Science Monthly Volume 17.djvu/34

24 10 shows a special application of the principle of the tasimeter devised at my suggestion by Professor C. A. Smith, of Washington University, St. Louis. A is a silver tube securely fastened at the top into a brass collar E. At the lower end, o, a steel rod, B, is firmly joined to the silver tube and runs up within it through the collar, E, and the carbon,



C, to a nut, D, by which the whole is clamped together. At E a screw-thread is cut, so that a brass tube, somewhat larger and longer than the silver tube, may be joined with the instrument for purposes of protection. The expansion or contraction of the silver tube, or, if the change in temperature is not sudden, the difference in expansion or contraction between the silver and the steel determines the variation of pressure on the carbon. It is proposed to use this instrument in determining the changes of temperature in steam cylinders, the laws of motion of a fluid whose temperature is not uniform, the rapidity of mixture when fluids of different temperatures are brought together, and the number of thermal units in any given volume of fluid.

After having discovered the peculiar properties of the carbon button, Mr. Edison made the current pass through several carbon disks instead of one. Increase in the intensity of the sound was noticed, but the articulation was impaired. The experiment was tried in a number of different devices, one of which is shown in Fig. 11. Instruments



of this class, whose object is to magnify the sound, have come to be known as microphones, though it is doubtful if any of them succeeded in transmitting very faint sounds so that they could be intelligible at a distance.

Intimately associated with Mr. Edison's discovery and use of the properties of the carbon button are the experiments of Professor Hughes, of London. In May, 1878, Professor Hughes made the following discovery: He took a short glass tube and filled it with white silver powder, a mixture of tin and zinc. The ends of the tube were