MEASUREMENT OF TEMPERATURE 



they can be inserted into small spaces and parts of small animals and plants ; 

 their thermal capacity is also very small and they can therefore equilibrate 

 rapidly without altering the temperature of small objects. 



A variety of metals can be used, and some typically useful ones (e.g. 

 platinum-rhodium systems) are biologically inert. With certain pre- 

 cautions, they are remote reading instruments. They are without hysteresis, 

 or other aging affects. 



The disadvantages are: Materials such as platinum, rhodium, are usually 

 expensive; for sensitive work, the voltages to be measured accurately are 

 minute, and a high-grade galvanometer is needed with attendant delicacy 

 and expense; a cold-junction reference temperature must be maintained 

 with an accuracy of an order better than the measurement being taken; 

 elaborate compensation precautions may be involved in the circuitry, or 

 it may be necessary to thermostat the galvanometer and remaining accessory 

 equipment. 



Practical considerations of the thermocouple — A closed loop of wire 

 containing a circulating current is not very much use; yet as soon as any 

 measuring device is placed in the circuit, containing, as it will, a number of 

 metals, such as copper, brass, constantin, etc., in series, we have a loop 

 containing many junctions {Figure 29.2). If each of these junctions con- 

 tributed their own Peltier effects, the original e.m.f. across the junctions 



^^^ r<2H<^i 



Figure 29.2 Figure 29.3 



could never be realized. The law of intermediate metals states that provided 

 the temperature remains unchanged any different metal, or series of metals, 

 may be placed in one part of the leads forming a pair of junctions without 

 changing the overall e.m.f. It is not only important to keep the temperature 

 of the measuring instrument constant for this reason — the voltages generated 

 by thermocouples are of the order of microvolts per degree — so that one 

 has to use a really sensitive instrument which will only give stable readings 

 if its temperature is kept reasonably constant; all its component parts and 

 the leads in the circuit will have temperature-sensitive resistances. It is 

 thus desirable to use massive copper leads, keeping down the additional 

 resistance of the circuit and also saving long leads of expensive materials 

 such as platinum. The meter should be protected by a shorting switch 

 which is opened to take readings. Figure 29.3 indicates the parts of the 

 circuit which should not undergo much temperature change. In addition, 

 leads and contacts must be kept thoroughly clean (e.g. copper contacts 

 should be scraped), and the humidity should not be too great, to prevent 

 surface films of water and voltaic cells at metal junctions. 



The typical order of voltage to be measured is 50 //V per degree C differ- 

 ence in junction temperatures. At first sight it seems that if this is displayed 

 on a 50 ^V meter, a reading to 0-0 1°C is quite straightforward, and a reading 



387 



