PRACTICAL METHODS FOR TEMPERATURE CONTROL 



to the required amount. Stability is of course provided by the overall negative 

 feedback from heater to thermistor, via the air in the enclosure. The appara- 

 tus works quite satisfactorily with power supplies merely rectified by germa- 

 nium rectifiers and smoothed by 32 [xF. If, however, the sensitivity control 

 is turned up so that the output stage is either turned 'on' or 'off' by movement 

 through a small temperature excursion, then the heater can be replaced by a 



+ 300 



2I<Q 

 Heater 



Figure 29.22 Circuit for ultra-fine control of air temperature in small 

 enclosures; the thermistor is of type 100 kQ, at 20°C. Two control ranges are 

 available from the switch which could also incorporate a background heater 



on the higher range 



powerful relay which will control large heat loads'. With this method, con- 

 trol of air temperature to 1/50°C in a litre of air in an uninsulated plastic box 

 has been obtained. 



Hot stages 



The temperature control of specimens usually immersed in liquids on the 

 microscope stage has received much attention. The greatest problem 

 associated is that of allowing the illumination of the specimen. Stages 

 using circulating water in a transparent cell, and electrical heating of a 

 perforated metal block, are in regular use. The important consideration 

 here is that of putting the specimen in contact with a large reservoir of heat 

 so that it can readily obtain the heat it itself loses by conduction or evapora- 

 tion. Wherever possible the specimen should be supported on the minimum 

 of insulating material, e.g. on a thin coverglass, and the surface which heats 

 it must be polished to ensure the maximum of contact. 



q.m.b. (page 394) means quick-make and -break, i.e. 'snap' or toggle action. — ED. 



411 



