PRACTICAL DETAILS OF ELECTRICAL METHODS 



will need little ventilation. The smallest wick is needed, together with a 

 tiny reservoir. The device can either be used calibrated against the direct 

 reading of the galvanometer or, alternatively, the galvanometer can be 

 zeroed by applying a current to a small heater wound within the wick, and 

 the machine calibrated against the heater current {Figure 30.1). The sensi- 

 tivity can be greatly increased by using a thermopile of many junctions 



Figure 30.1 Compensated wet bulb system for tfiermocouple hygrometer. 

 The wet bulb {w) has a small heater coil wound within its wick, and the current 

 in it is adjusted until the galvanometer (g) is zeroed, readings are then made 



off the milliamnieter 



(see Chapter 29) for the two sets of junctions need not be greatly separated 

 in distance. 



Resistance thermometers 



If the wet and dry bulbs are two resistance thermometers which form two 

 arms of a bridge current, the out-of-balance will indicate a function of 

 humidity; the law will be complex, and temperature dependent, but if a 

 second similar bridge is so arranged that it contains a 'dry bulb' with a fixed 



Figure 30.2 Schematic layout for controlling the voltage on a wet {WB) and 

 dry (DB) resistance thermometer bridge, using a second dry bulb element 



resistance in place of the wet bulb element, a suitable comparison of the 

 outputs of the two bridges can be made to give a reasonable measure of 

 humidity, independent of temperature, for the second dry bulb compensates 

 out the first one. This can either be done by a cross-coil meter movement, 

 which presents the resultant of the two bridge generated currents, or by using 

 the output of the compensator bridge as the feed of the primary bridge, in 

 which case the out-of-balance will give a direct measure of saturation 

 deficiency {Figure 30.2). 



415 



