140 



BIOLOGICAL EFFECTS OF RADIATION 



of black-body radiation at 2727° K. has been made available. This 

 higher temperature promises to make practicable standardization in the 

 region of shorter wave-lengths by direct comparison with a black body. 

 A brief description of this type of source therefore seems desirable. 



The arrangement consists of a crucible containing the platinum (or 

 iridium) in which is immersed a small tube which serves as the actual 

 source (see Fig. 4). The crucible and immersed tube are made of fused 

 thorium oxide (thoria), ground to a fine powder and then molded to 



I m 



UJ 



T T 



. Platinum 



. s^ Un fused 

 Thona 



fusecf Thoria 



S^^^^^ 



Fig. 4. — Diagram of standard black body and collimating system. 



of Standards.) 



{National Bureau 



shape, using thorium chloride as a binder. It has been found that such 

 crucibles, even after extensive use, do not contaminate metals of the 

 platinum group. Powdered thoria is used for insulation. A small 

 quantity of powdered thoria is placed in the lower portion of the tube 

 to conceal the colder bottom of the tube. 



The metal is melted by means of a high-frequency induction furnace. 

 After the metal has been heated a few degrees above its melting point, the 

 power is cut off. Successive observations of the radiation show decreas- 

 ing intensity until the metal begins to freeze, a constant intensity during 

 the period of freezing, and again a falling intensity as the solid metal 



