178 



ELECTRICAL MEASUREMENTS 



trol the current in a second circuit. If the voltage in the base-collector 

 circuit is larger than that in the emitter-base circuit, the control amounts 

 to an amplification. 



P-type N-type 



Collector 



(0) 



(b) 



Fig. 13-3. Solid state (semiconducting) devices: (a), a junction diode; (b), 

 N-P-N transistor. For explanation, see text. 



In practice, the N-P-N transistor can be used in a circuit in any of 

 several ways. The P-N-P transistor is the same kind of device except that 

 the polarity is reversed— positive poles becoming negative and vice versa 

 —so that the current carriers through the base are electrons rather than 

 holes. 



Transistors have several decided advantages over vacuum tubes. Since 

 they do not depend upon heaters, they can be operated with very little 

 applied power. They also can be made very tiny, which helps to reduce 

 the size and weight of instruments. Generally, they have a much longer 

 life than vacuum tubes. Other advantages, and some disadvantages, will 

 be discussed later under Amplifiers. 



Electronic systems 



An electronic system, as the term is used here, is a device or com- 

 bination of devices that responds to some change in the environment in 

 a characteristic way to produce an electrical change in a measuring in- 

 strument or to bring about some control over the environmental change. 

 Systems of this sort are adaptable to measurement of a variety of bio- 

 logical phenomena. Some biological reactions produce voltages directly, 



