442 Resonance Experiments with the Longest Heat- Waves. 



The diameters are as follows on the plates photographed : 

 fig. 1 f-005 mm.), fig. 2 (-01 mm.), fig. 3 ('02 to -03 mm.), 

 fig. 4 >03 to -04 mm.), fig. 5 (-05 mm.), fig. 6 ('06 to 

 'OS mm.) . Thus the smallest particles used had a diameter 

 equal to about 1/20 of a wave-lengtb, and the largest 2/3 of 

 a wave-length. The transmission of each plate of resonators 

 was compared with that of a clean plate of quartz of the same 

 thickness, for radiation of wave-length 1 yu,, and the 112 jx 

 waves obtained by focal isolation. The transmission of the 

 1 fju waves gives us a rough measure of the ratio of the 

 covered to the uncovered portion of the surface, for the metal 

 droplets act merely as opaque obstacles for these very short 

 heat-waves. The observations were made with Dr. Pfund's 

 infra-red spectrometer, which he kindly placed at my dis- 

 posal. The transmission of the 112 jul waves was measured 

 with a radiomicrometer of the same type as the one used in 

 the experiments carried out in collaboration with Professor 

 Rubens. The instrument was made for me by Herr Obst, 

 the mechanician of the Berlin Physical Institute, but as it 

 arrived with the thermo-electric junction damaged it was 

 fitted up with a much lighter junction, which Dr. Pfund 

 made for me, with perhaps 1/10 of the heat capacity of the 

 original one. It proved far more sensitive than the one used 

 in the earlier work, giving about double the deflexion for 

 the same period, with the 112 //, waves obtained under 

 identical conditions. The transmissions of a number of 

 resonator plates are given in the following table : — 





Percentage 



transmitted of 



Radiation X = l ju. 



Percentage 



transmitted of 



Radiation A = 112 fx. 



Diameter of 

 Spheres. 



Fig. 1. 



45 





100 





•005 mm. 



Fig. 2. 

 Fig. 3. 

 Fig. 4. 



26 

 32 

 45 





95 



82 

 58 





•01 



•02--03 



•03--04 



Fig. 5. 

 Fig. 6. 



26 



30 





50 



27 





•05 

 •07--08 



This table shows us that the droplets have no effect on the 

 very long heat-waves until their diameter exceeds about 1/10 

 of a wave-length. When the diameter is about J A, as in fig. 4, 

 the transmission is not much greater for the 112 \x waves 

 than for the short ones. For the case shown in fig. 5 we 

 have slightly larger spheres packed much closer together 

 however, so that we have only 26 per cent, transmission for 

 short waves ; even now 60 per cent, of the energy o£ the 

 112 fx radiation is passed by the plate. On a further increase 

 of size, fig. 6, the transmission is about the same for both 

 wave-lengths. 



