502 T. Mizuno on Tinfoil Grating as a 



The difference between the results of this and those of the 

 last experiment I am inclined to ascribe to the diffraction 

 effects of electric waves caused by the plate. 



Experiment 8. 



A rectangular wooden board, 69 centim. in length and 

 14 centim. in breadth, was coated all over with tinfoil. When 

 it was held vertically with the longer edges parallel to the 

 primary oscillations, the resistance of my tinfoil grating 

 placed behind it showed no change, but when the edges 

 were placed perpendicular to the primary oscillations, I 

 found : — 



Experiment. 



Initial 

 Resistance. 



Final 

 Resistance. 



Decrease 

 of Resistance. 



1. 



2. 

 3. 



1300 

 130-8 

 131-4 



1270 

 127-7 



127-8 



3-0 

 3-1 

 36 



This shows that the intensity of the diffraction effect de- 

 pends on the extension of the tinfoil board with respect to the 

 direction of the primary oscillations. 



Experiment 9. 



A parabolic cylinder exactly similar to the one mentioned 

 before was placed against the latter, at a distance of about 

 1*5 metre, so that their focal lines should be parallel. My 

 tinfoil grating No. 2 was held in the focal line of the second 

 parabolic cylinder and tested ; thus : — 



Experiment. 



Initial 

 Resistance. 



Final 

 Resistance. 



Decrease 

 of Resistance. 



1. 

 2. 

 3. 



2299 

 229-9 

 229-9 



2190 

 219-7 

 219-6 



10-9 

 10-2 

 10-3 



