Masterpieces of Science 



electrical instruments, the incoming flash causing 

 a tiny heap of carbon dust to provide it with a 

 path through which it could safely pass to the 

 earth. Professor Temistocle Calzecchi Onesti of 

 Fermo, in 1885, in an independent series of re- 

 searches, discovered that a mass of powdered 

 copper is a non-conductor until an electric wave 

 beats upon it; then, in an instant, the mass re- 

 solves itself into a conductor almost as efficient 

 as if it were a stout, unbroken wire. Professor 

 Edouard Branly of Paris, in 1891, on this princi- 

 ple devised a coherer, which passed from resist- 

 ance to invitation when subjected to an electric 

 impulse from afar. He enhanced the value of 

 his device by the vital discovery that the con- 

 ductivity bestowed upon filings by electric dis- 

 charges could be destroyed by simply shaking 

 or tapping them apart. 



In a homely way the principle of the coherer is 

 often illustrated in ordinary telegraphic practice. 

 An operator notices that his instrument is not 

 working well, and he suspects that at some point 

 in his circuit there is a defective contact. A little 

 dirt, or oxide, or dampness, has come in between 

 two metallic surfaces; to be sure, they still touch 

 each other, but not in the firm and perfect way 

 demanded for his work. Accordingly he sends a 

 powerful current abruptly into the line, which 

 clears its path thoroughly, brushes aside dirt, 

 oxide, or moisture, and the circuit once more is as 

 it should be. In all likelihood, the coherer is 

 acted upon in the same way. Among the phy- 

 114. 



