Detector for Electric Waves. 447 



to show that the tinfoil strip was somewhat stretched along 



its edge by the act of cutting it. Non-sensitive 



gratings showed none of these characteristic Fig. 3. 



appearances, but had the gap between the strips I /a , 



much wider, with no decided slope and no Lrn^ L^ 



appreciable folds along the edges of the strips. 



Hence for a grating to be sensitive, it appears to be necessary 



that the gaps should be narrow and their margins sloped and 



in folds. 



§ 5. Although I have been unable to see clearly the interior 

 of a gap, yet it is quite reasonable to assume that in sensitive 

 gratings there will be numbers of leaflets along the margins 

 of adjacent tinfoil strips ; and the existence of such leaflets 

 once admitted, the explanation of the action of the gratings 

 becomes clear. For in a properly constructed grating some 

 of the leaflets may easily come in contact with one another 

 under the action of the electric waves, because of the extremely 

 small distance between any two opposite leaflets in the narrow 

 gap. Then, too, it seems to me that these leaflets must be of 

 various dimensions and, accordingly, some of them will be 

 extremely sensitive, others less so but still highly sensitive, 

 others again only moderately so. This being the case, the 

 amount of change in the resistance of the grating must depend 

 upon the intensity of energy of the impinging electric 

 oscillations, for, when it is not great enough, only the most 

 sensitive leaflets will come into play, but when it is sufficiently 

 great all the effective leaflets will be brought into action. 

 All the experiments I have yet made are in agreement with 

 this representation of the matter. 



§ 6. A grating, well prepared so as to fulfil the conditions 

 mentioned above, proves to be an extremely sensitive detector 

 for electric waves, as will be seen from the experiments which 

 I now describe. 



Experiment 1. A Hertzian parabolic vibrator, ABC, was 

 placed horizontally with aperture turned upwards, as shown 

 in fig. 4. The aperture was 

 covered with a sufficiently large Fig. 4. 



wooden plate, ADC, entirely 

 coated with tinfoil. A grating, 

 whose initial resistance was about 

 71 ohms, was placed at about 

 5 centim. from the plate and in 

 a vertical line with the primary 

 conductor, 0, radiating electric 

 waves of 60 centim. wave-length. 

 Then, exciting the primary oscillations, I always found that 



