18 



PROCEEDINGS OF THE NATIONAL MUSEUM 



The system at the Smithsonian incorporates a telethermometer 

 that utilizes a thermistor probe as a sensmg element and provides 

 temperature readings directly from a temperature scale. The tele- 

 thermometer's range is from —40° to 150° C. Both surface and 

 air probes are used and the probe leads are connected through a 

 multiple-selector switch to permit convenient temperature reading 

 at several places. 



Figure 7. — Simple freeze-dry apparatus (.:/= specimen chamber; 5= commercial deep- 

 freeze; C= recompression valve; Z)= vacuum pump; £== wood-spacer insert). 



Recorder. — Any versatile general-purpose D.C. voltage and cur- 

 rent recorder may be used to record temperatures continuously. 

 One with a low-to-high-level range (200 microvolts to 500 volts and 

 200 millimicroamperes to 100 microamperes) is suitable for freeze- 

 dry purposes. It is important that the recorder be compatible with 

 the temperature device in scale range and voltage output. 



Construction of a Freeze-dry System 



The simplest apparatus suitable for freeze-drying consists of a 

 commercial deep-freeze containing a small chamber connected to a 

 gas-ballasted vacuum pump (fig. 7). 



