THE MAGNETIC RADIOMETER 179 



for the determination of the different intensities of radiation 

 at the time and place of observation almost simultaneously 

 with that of photosynthesis. This is secured by throwing 

 the particular rays first on to the plant-vessel and then on 

 to the Radiometer. We thus secure the three elements 

 necessary for quantitative research : (1) the wave-length of 

 the rays, (2) their energy of radiation, and (3) the photo- 

 synthetic activity induced by each particular ray. 



The Magnetic Radiometer 



I have succeeded in directly determining the energy 

 of the different rays by measuring the elongation of a metallic 

 wire coated with lamp-black. The particular spectral ray 

 falling on the wire is absorbed, and thus raises the 

 temperature proportionately to its energy of radiation. 

 The rise of temperature is excessively feeble, being of the 

 order of TT7 o\ooo° C. > tne resulting increase of length is so 

 minute as to be undetectable by any method of magnifica- 

 tion which has been hitherto employed. I have, however, 

 got over thi*s difficulty by means of a magnetic device by 

 which it is possible to obtain a magnification of about 

 50,000,000 times or even higher. 



Fig. 49 is a diagrammatic representation of the instru- 

 ment, w is the strip or wire which becomes lengthened by 

 rise of temperature produced by the absorbed radiation. It 

 is attached by a hook to the short arm of a long magnetic 

 lever, the n end of which is lowered by any elongation of the 

 sensitive wire. In front of the n end of the magnetic lever 

 is suspended a small magnetic needle with an attached 

 mirror. As the n pole of the magnetic lever is lowered 

 it produces increasing deflection of the suspended needle, 

 which is magnified by the spot of light reflected from the 

 attached mirror. The sensitiveness of the apparatus is 

 very greatly enhanced by the employment of a perfect 

 system of astatic needles. 



This method for measurement of radiation is extra- 



