EDISON'S ACOUSTIC INVENTIONS. 



139 



The Tasimetek. — Fig. 32 shows in perspective the latest form of 

 the Edison microtasimeter, or measurer of infinitesimal pressure. The 

 value of the instrument lies in its 

 ability to detect small variations 

 of temperature. This is accom- 

 plished indirectly. The change 

 of temperature causes expansion 

 or contraction of a rod of vulcan- 

 ite, which changes the resistance 

 of an electric circuit by varying 

 the pressure it exerts upon a 

 carbon -button included in the 

 circuit. During the total eclipse 

 of the sun, July 29, 1878, it suc- 

 cessfully demonstrated the exist- 

 ence of heat in the corona. It is 

 also of service in ascertaining the 

 relative expansion of substances 

 due to a rise of temperature. 



In Fig. 33 the important parts 

 are represented in section, affording an insight into its construction and 

 mode of operation. 



The substance whose expansion is to be measured is shown at A. 

 It is firmly clamped at ^, its lower end fitting into a slot in the metal 



Fig. 32. 



plate, M, which rests upon the carbon-button. The latter is in an elec- 

 tric circuit, which includes also a delicate galvanometer. Any variation 



