MEASUREMENTS 



39 



Several electrical devices also measure temperature. A thermocouple 

 is formed by joining wires of two different kinds. For example, a piece 

 of copper wire could be joined to a piece of wire of the alloy called 

 constantan to form a loop half copper and half constantan. If one junc- 

 tion is at a higher temperature than the other, a small but measurable 

 voltage will exist in the circuit, the magnitude of the voltage depending 

 on the temperature difference. 



A resistance thermometer measures temperature by detecting changes 

 in the resistance of a platinum wire. The platinum wire can be used 

 as one of the resistors in a Wheatstone bridge (Fig. 4-1), and tempera- 

 tures can be measured with great precision. A somewhat similar device 

 called a thermistor depends upon changes in resistance of a semiconduct- 

 ing material. Because thermistors offer the advantage that the detecting 

 element can be extremely small, they are particularly useful in some 

 biological experiments. Thermistors have made it possible to measure 

 temperatures in such improbable places as stomachs. Several thermistor 

 probes are illustrated in Fig. 4-3. 



Fig. 4-3. Several thermistor probes, each of which can measure temperature. Size 

 can be estimated from the wires at the bottom, each about 3 mm in diameter. 

 (Courtesy Yellow Springs Instrument Co.) 



