45° POPULAR SCIENCE MONTHLY. 



water adhering to the metallic particles through which electrolytic 

 action occurs, 



A good metallic filings tube for use as a receiver in Hertzian wave 

 telegraphy should exhibit a constancy of action and should cohere and 

 decohere^ to use the common terms, sharply, at the smallest possible 

 tap. It should not have a current passed through it by the external 

 cell of more than a fraction of a milliampere, or else it becomes 

 wounded and unsensitive. 



The investigations which have already taken place seem to show 

 pretty clearly that the agency causing the masses of filings to pass 

 from a non-conductive to a conductive condition is electromotive force, 

 and that therefore it is the electromotive force set up in the aerial by 

 the incident waves which is the effective agent in causing the change 

 in the metallic filings tube, when this is used as a telegraphic kuma- 

 scope. This transformation of the tube from a non-conductor to a con- 

 ductor is made to act as a circuit-closer, completing the circuit, by 

 means of which a single cell of a local battery is made to send current 

 through an ordinary telegraph relay, and so by the aid of a second 

 battery operate a telegraphic printer or recorder of any kind. Hence, 

 it is clear that after one impact, the metallic filings tube has to be 

 brought back to its non-conductive condition, and this may be achieved 

 in several ways. (1) By the administration of carefully regulated 

 taps or shocks or by rotating the tube on its axis; (2) by the aid of 

 an alternating current; (3) in those cases where filings of magnetic 

 metals are employed, by magnetism. 



The decoherence by taps was discovered by Branly,* and Popoff, 

 following the example of Sir Oliver Lodge, employed an electric bell 

 arrangement for this purpose, f 



Mr. Marconi, in his original receiving instruments, placed an elec- 

 tromagnet under the coherer tube with a vibrating armature like an 

 electric bell. J This armature carries a small hammer or tapper, which, 

 when set in action, hits the tube on the under side, and various 

 adjusting screws are arranged for regulating exactly the force and 

 amplitude of the blows. This tapper is actuated by the same current 

 as the Morse printer, or other telegraphic recorder, so that when the 

 signal is received and the metallic filings tube passes into the con- 

 ductive condition and closes the relay circuit, this latter in turn closes 

 the circuit of the Morse printer or other recorder, and, at the same 

 time, a current passes through the electromagnet of the tapper and 

 the tube is tapped back. This sequence of operations requires a certain 



* See The Electrician, Vol. XXVII., 1891, p. 448. 



t Journal of The Russian Physical and Chemical Society, Vol. XXVIII. j 

 division of physics. Part I., January, 1896. 



t See Brit. Patent Specification, No. 12,039, June 2, 189G. 



