158 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



ics, that permits him to appraise the possibilities of the heat engine ; 

 and it tells him, for example, that the enormous reservoir of heat 

 which the earth's atmosphere constitutes is not available for use in a 

 heat engine. This same second law of thermodynamics states that, 

 in the act of collecting sunlight and converting it to heat at a lower 

 temperature level, a degradation of solar energy has occurred; the 

 energy has been made less available for conversion to power even 

 though none of it has been lost; and no process — no matter how 

 clever the inventor — can restore the energy to a form as intrinsically 

 useful as when it arrived here as solar energy just before its conver- 

 sion to heat. 



In consequence of this important limitation on what can be ex- 

 pected so long as one's interest is restricted to heat engines, it is 

 appropriate to consider other means of conversion of solar energy to 

 power which do not involve as a first step the collection of the energy 

 as heat, but which instead make use of the special nature of the 

 energy as it arrives. Solar energy reaching the earth consists of a 

 jumbled mass of radiations of wave lengths varying from the short 

 ultraviolet through the visible spectrum and out into the infrared, 

 roughly one-third of the total energy lying in the visible spectrum. 

 The radiation might be likened, if the analogy is not pushed too far, 

 to a shower of bullets — unit quantities of energy, known as quanta, 

 each of a particular wave length. The quanta of shortest wave 

 lengths have the greatest unit energy content ; and almost two-thirds 

 of the total energy consists of relatively impotent quanta in the 

 infrared. If, instead of pouring all these quanta into the funnel of 

 a heat engine, they are given a chance to show their individuality, 

 what are their specialties? One, of particular interest to us at pres- 

 ent, is the phenomenon of photoelectricity, the ability of light quanta 

 of certain wave lengths to knock electrons out of atoms or atomic 

 lattices in crystals and produce an electric current. 



Many of you have encountered this phenomenon in using that type 

 of camera exposure meter which indicates on a dial the intensity of 

 illumination. Light is there being converted into electrical energy 

 which is in turn used to make the galvanometer needle move. The 

 light-sensitive unit of such a device is one of two kinds, each referred 

 to as a blocking-layer photocell. The copper oxide cell is typical ; it 

 consists of a massive plate of copper which has been oxidized on one 

 face and then etched, to produce thereby a layer grading from 

 cuprous oxide through all proportions of oxygen down to pure cop- 

 per. The cuprous oxide surface is covered with a thin film of an- 

 other metal, so thin as to be transparent to light quanta. There is 

 thus produced a sandwich in which the outer layers are metal and 

 the inside layers consist of material graded in character in a direc- 

 tion normal to the surface. If a quantum of visible light strikes 



