Detector for Electric Waves. 449 



will now be quite clear from the results of the above experi- 

 ments. It is next of great importance to describe some 

 experiments as to the variation of the sensibility. In its 

 primitive state, the grating properly constructed is so sensi- 

 tive that it can detect even the smallest electric oscillations. 

 But after having been used a few times, its sensibility under- 

 goes a sudden and decided diminution, and then remains 

 nearly constant. At first, when the grating is exposed to 

 electric waves and its resistance consequently diminished, a 

 single tap given to it is almost enough to restore the resistance 

 to its initial or primitive value. But when we have used the 

 grating repeatedly, we find it necessary to give it a greater 

 number of taps to effect this restoration. Later on, when the 

 sensibility has diminished to a certain value, it seems to retain 

 that value without any decided further change for a long time. 

 This variation in the sensibility may be accounted for in the 

 following way : — As mentioned in § 5, the effective leaflets 

 along the margins of the several tinfoil strips may be of 

 different sizes, and some of them possibly very small. The 

 smaller the leaflets the more sensitive to electric disturbances, 

 and consequently the more liable to fatigue will they be. 

 Hence in the primitive state such leaflets are easily affected 

 by even very weak electric impulses, but soon lose this 

 sensibility as a result both of the repeated electric distur- 

 bances and of the mechanical taps given to them each time. 



§ 8. Though the sensibility of the grating thus always 

 diminishes to a certain extent by a little use, still it is even 

 in such a state far superior to that of an ordinary Hertzian 

 resonator. Even where the latter fails, the grating always 

 shows the presence of electric waves if there be any. Experi- 

 ments on the nature of electric waves, namely, on rectilinear 

 propagation, reflexion, refraction, diffraction, polarization, &c, 

 can all be easily carried on by means of a properly constructed 

 grating. Moreover, such a grating gives not only qualitative, 

 but also quantitative results, to a certain extent, because the 

 amount of diminution of the resistance depends upon the 

 quantity of energy of the impinging waves. Hence, I believe, 

 it may prove to be of great advantage to make use of such 

 gratings in all lecture experiments as well as in laboratory 

 researches on electric waves. 



In conclusion I wish to express my thanks to Mr. U. 

 Takashima for the kind and earnest assistance he has given 

 me in the preparation of many of these gratings and in carry- 

 ing out researches upon them. 



Phil Mag. S. 5. Vol. 4L No. 252. May 1896, 2 I 



