The Wireless Telegraph 



a nearly closed circle of wire, the ends being 

 soldered to metal balls almost in contact. With 

 this simple apparatus he demonstrated that 

 electric waves move with the speed of light, and 

 that they can be reflected and refracted pre- 

 cisely as if they formed a visible beam. At a 

 certain intensity of strain the air insulation broke 

 down, and the air became a conductor. This 

 phenomenon of passing quite suddenly from a 

 non-conductive to a conductive state is, as we 

 shall duly see, also to be noted when air or other 

 gases are exposed to the X ray. 



Now for the effect of electric waves such as 

 Hertz produced, when they impinge upon sub- 

 stances reduced to powder or filings. Conductors, 

 such as the metals, are of inestimable service to 

 the electrician ; of equal value are non-conductors, 

 such as glass and gutta-percha, as they strictly 

 fence in an electric stream. A third and re- 

 markable vista opens to experiment when it deals 

 with substances which, in their normal state, are 

 non-conductive, but which, agitated by an elec- 

 tric wave, instantly become conductive in a high 

 degree. As long ago as 1866 Mr. S. A. Varley 

 noticed that black lead, reduced to a loose dust, 

 effectually intercepted a current from fifty 

 Daniell cells, although the battery poles were 

 very near each other. When he increased the 

 electric tension four- to sixfold, the black-lead 

 particles at once compacted themselves so as to 

 form a bridge of excellent conductivity. On this 

 principle he invented a lightning-protector for 

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