182 ELECTRICAL MEASUREMENTS 



typical cadmium selenide photoconductive cell is about the size of a 

 small pea. These transducers have some disadvantages, but for certain 

 applications in experimental research they can be very useful. 



+ I00v 4-300V +500v 



^^^. 



Photocathode +200v +400v +600v 



Fig. 13-6. Multiplier Phototube. A quantum 

 of radiant energy causes the emission of an 

 electron from the photocathode. At the first 

 dynode (+100v) and at succeeding dynodes, 

 additional electrons are liberated by secondary 

 emission. 



Combinations: In many instances, some combination of transducers can 

 be used. Although a very small movement might be detected directly 

 w^ith a strain gage, it might sometimes be more convenient to measure 

 such a movement by observing its effect on a beam of light. A sensitive 

 galvanometer responds to a small current by moving a suspended coil of 

 wire within a magnetic field. A small mirror, rotated along with the 

 coil, reflects a beam of light which moves across the scale as the coil 

 turns. The small mirror has less inertia than the needle in an ordinary 

 meter, so the galvanometer responds to smaller currents. Thermocouples 

 respond to temperature differences, but if one of the junctions is black- 

 ened it will absorb light and become warmer and thus will measure ra- 

 diant energy. A scintillation counter is a derived instrument because 

 ionizing radiation causes flashes of light in the phosphor, and these flashes 

 are detected by the multiplier phototube. 



Output transducers 



Of the many types of output transducers, the galvanometer was the 

 earliest used. If a coil of wire is suspended between the poles of a magnet, 

 any current in the wire tends to deflect the coil transversely across the 

 magnetic field. A typical measuring meter consists of a coil mounted on 

 jeweled pivots surrounded by the magnet. The torque or rotating force 

 in any given meter is proportional to the current through the coil. 

 An attached pointer moves across a scale, so current can be read di- 



