ELECTRICAL MEASUREMENTS 181 



bridge. Changes in resistance can be amplified and used to drive a re- 

 corder. 



Temperature Transducers: The thermocouple, the resistance ther- 

 mometer, and the thermistor were discussed in Chapter 4. Each of these 

 is properly called an "input transducer." The thermocouple produces a 

 d-c voltage directly, while the other two instruments give variations in 

 resistance. In either case, it is not difficult to fit the transducer into an 

 electrical system. 



Radiation Transducers: Probably transducers which respond to radiant 

 energy occur in the greatest variety. This is true partly because light is 

 energy, and within the visible and ultraviolet range the quanta are large 

 enough to cause measurable electrical or chemical effects. The photo- 

 voltaic cell, or "barrier-layer cell," is a semiconducting device. Quanta of 

 light displace electrons within the crystal. With a proper arrangement 

 of semiconducting and conducting materials, the continued displacement 

 of electrons becomes a small current which can be measured. Voltages 

 always are low, but the currents are large enough to be measured on 

 sensitive meters. Nearly all photoelectric exposure meters used in photog- 

 raphy are barrier-layer cells. 



A phototube is a vacuum tube in which the cathode is sensitive to 

 light. Quanta of radiant energy impinging on the cathode release elec- 

 trons which are drawn to the plate or anode. Since the current across the 

 tube is dependent on the number of quanta striking the photoemissive 

 surface in a unit time, the output of the tube is proportional to the light 

 intensity. 



A multiplier phototube is a phototube with a built-in amplifier called 

 an electron multiplier. A series of dynodes (electrodes with increasing 

 positive charge) is arranged so that electrons liberated from the photo- 

 emissive cathode are drawn to the first dynode. Each electron causes the 

 emission of additional electrons from the dynode surface. As shown in 

 Fig. 13-6, a combination of eight or nine dynodes can result in a con- 

 siderable multiplication of the current. Multiplier phototubes respond 

 rapidly to very dim light or to quite small changes in light intensity. 

 These tubes have been incorporated in a number of standard laboratory 

 instruments such as spectrophotometers. 



Several semiconducting materials can be used effectively as light de- 

 tectors because of a large decrease in resistance upon illumination. A 

 dozen or more materials are available. Lead sulfide is used to measure 

 infrared radiation in several of the commercial spectrophotometers. A 



