Figure 24. Wood shelter which houses the hygro thermograph. 



a banjo-spread human hair element which expands when the humidity increases and 

 contracts when it decreases, causing the second recording pen to deflect appro- 

 iately. In addition to the daily observations, a 7-day continuous chart record 

 is produced. 



e. Maximum and Minimum Thermometers . The NWS thermometers (housed in the 

 instrument shelter) are used to determine the daily extreme temperatures (Fig. 

 24). A constriction in the capillary of the maximum thermometer prevents the 

 mercury from flowing back into the bulb. The mercury is forced past the con- 

 striction as the temperature rises but does not return through it when the tem- 

 perature falls, causing a thread of mercury to remain in the tube at , the highest 

 temperature reached. The thermometers are held in a special mounting device 

 called a Townsend Support (see Fig. 23) which allows a reset of the thermome- 

 ters without removal. The high temperature thermometer is reset by spinning it 

 in the Townsend Support, thereby causing enough centrifugal force to force the 

 mercury back through the constriction. 



The minimum thermometer has a small, dark glass index in the thermometer 

 bore which is pulled toward the bulb by the surface tension of the mercury 

 meniscus as the temperature drops and remains stationary when the temperature 

 rises. The thermometer is mounted horizontally except when the instrument is 

 reset. Resetting is facilitated by the Townsend Support and consists of rais- 

 ing the bulb of the thermometer, thus causing the index to slide downward (to- 

 ward higher temperatures) in the tube. The thermometers are read and reset 

 once a day. Additional details on the thermometers can be provided by the 

 Basic Observations Branch, NWS. 



27 



