438 



ALTERNATING CURRENTS 



The brightness of Si and of S 2 depends on the intensity of the 

 source which illuminates each. The light from *Si and S 2 is re- 

 flected by the plane mirrors MI and M 2 to the total reflecting 

 prisms PI and P 2 . The hypotenuses of the two prisms are in 

 contact over a circular area only. The light striking this area of 

 contact can pass through; other light is totally reflected. That 

 is, only the central beam, shown dotted, from the mirror MI, 

 passes through to the eye piece. The remainder of the light is 

 turned away. On the other hand, the central beam shown by 

 a solid line, from M 2 passes through the center circle and is 

 absorbed by the walls of the box. The remaining light is re- 

 flected to the eye piece. The observer sees two distinct ellipses 

 if the photometer is out of balance. (The circle of contact of 

 the two prisms appears as an ellipse to the observer because he 

 is viewing the circle at an angle.) When the two ellipses blend, 

 the same illumination is coming from each source and the pho- 

 tometer is in balance. To eliminate any errors due to differences 

 in the two sides of the screen Si and *S 2 , the photometer screen 

 should be reversed. 



PORTABLE PHOTOMETERS 



201. The Sharp-Millar Photometer. A portable photometer 

 is necessary for making such photometric measurements as can 



FIG. 394. Sharp-Millar photometer. 



not be made in the laboratory, as on arc lamps in service, street- 

 lighting units, store lighting, etc. 



Portable photometers involve the same principles as laboratory 

 photometers. The difference lies in the compactness and in the 

 ease of manipulation. 



