HERTZIAN WAVE WIRELESS TELEGRAPHY. 447 



The logical development of the single contact is therefore the 

 infinite number of contacts existing in the tube of metallic filings, 

 which has been the form of kumascope most used for many years. In 

 its typical form it consists of a tube of insulating material with 

 metallic plugs at each end, and between them a mass of metallic 

 powder, filings, borings, granules or small spheres, lightly touching 

 one another. Imperfect contact must be arranged by light pressure, 

 and in the majority of cases the resistance is very large until an 

 electric wave falls upon the tube, when it drops suddenly to a small 

 value and remains there until the tube is given a slight shake or the 

 granules disturbed in any way, when the resistance suddenly rises 

 again. This type of responder is a non-restoring kumascope, and re- 

 quires the continual operation of some external agency to keep it in a 

 condition in which it is receptive or sensitive to electric waves. 



Much discussion and considerable research have taken place in con- 

 nection with the action and improvement of these metallic powder 

 kumascopes. As regards materials, the magnetic metals, nickel, iron 

 and cobalt, in the order named, appear to give the best results. The 

 noble metals, gold, silver and platinum, are too sensitive, and the very 

 oxidizable metals too insensitive, for telegraphic work, but an admix- 

 , ture may be advantageously made. 



Omitting the intermediate developments of invention, it may be 

 said that Mr. Marconi was the first to recognize that to secure great 

 sensibility in an electric wave detecter of this type the following con- 

 ditions must be fulfilled: An exceedingly small mass of metallic 

 filings must be placed in a very narrow gap between two plugs, the 

 whole being contained in a vessel which is wholly or partly exhausted 

 of its air, Mr. Marconi devoted himself with great success to the 

 development of this instrument, and in a very short time succeeded 

 in transforming it from an uncertain laboratory appliance, capable of 

 yielding results only in very skilled hands, into an instrument certain 

 and simple in its operations as' an ordinary telegraphic relay. He did 

 this, partly by reducing its size, and partly by a most judicious selec- 

 tion of materials for its construction. As made at present, the Mar- 

 coni metallic filings tube consists of a small glass tube, the interior 

 diameter of which is not much more than one eighth of an inch, which 

 has in it two silver plugs which are beveled off obliquely. These are 

 placed opposite to each other so as to form a wedge-shaped gap, about 

 a millimeter in width at the bottom and two, or at most three, milli- 

 meters in width at the top (see Fig. 1). The silver plugs exactly fill 

 the aperture of the tube, and are connected to platinum wires sealed 

 through the glass. The tube has a lateral glass tube fused into it, by 

 which the exhaustion is made, which is afterwards sealed off, and this 

 tube projects on the side of the wider portion of the gap between the 



