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THE POPULAR SCIENCE MONTHLY. 



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, 



Fio. 10. 



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 but- 

 ton, 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. Instru- 



ments 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 suc- 

 ceeded in transmitting very faint sounds so that they could be intelli- 

 gible 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 fol- 

 lowing 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 



