l8o CHAP. XXI. THE ENERGY OF THE SPECTRAL RAYS 



ordinarily sensitive. For instance, there is an apparatus 

 permanently adjusted in the Institute in which the sensitive 

 element is a thin strip of ebonite which is enclosed in a 

 tube with a narrow slit in front. When any person walks 

 at some distance past the instrument, the indicating spot of 



Fig. 49. The Magnetic Radiometer 



sn, magnetic rod supported on fulcrum ; short arm of magnet 

 attached to sensitive strip of metal w. Elongation of strip 

 lowers the n end, which causes increasing deflection of the 

 suspended needle with attached mirror M. Deflection 

 magnified by reflected spot of light. 



light remains perfectly quiescent until the individual is in 

 the line of sight of the sensitive strip ; a sudden deflection 

 is then produced by the radiation which is emitted from the 

 human body through thick warm clothing and a heavy 

 overcoat ; when he walks past the line of sight the indicating 

 spot of light returns to the original zero. 



For our present purpose such a high sensitiveness is 



