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Fic. 1—General diagram of the hail tunnel; (a) blower; (b) test object; (¢) air-cooler or vaporizer; 
(d) refrigerating compressor; (e) ammonia condenser; (f{) heater; (g) electrostatic filter; (h) point where 
humidity and ice-forming nuclei are injected; (1) section where the flow achieves homogeneity; (I) 
measuring section; (1) control panel; (m) liquid separator; (n) aleohol-water cooler for cooling the cold 
compressor 
perimental atmosphere. The electrostatic filter g, 
which has particular advantages with regard to 
ice-forming nuclei, serves to purify the air. (The 
special characteristics of the electrostatic filter 
have been discussed, for instance, by List and de 
Quervain (1956].) At the point h, humidity is con- 
tinuously added to the air which since leaving the 
blower has been dried, cooled, and cleaned. It is a 
matter of choice whether the humidity is intro- 
duced via a steam generator (Fig. 3), an air-water 
compressor (Fig. 2), or a rotor atomizer (Fig. 4). 
By varying the method of producing the droplets 
their size can be altered to quite extreme limits. 
At the same point in the tunnel h, nucleating sub- 
stances are added (Fig. 4), after having been va- 
porized in a high-temperature oven. The next ver- 
tical section of the tunnel i, which follows the 
humidity injection point, allows the air current 
to become less turbulent and the temperature of 
the droplets to adjust to the temperature of the 
air about them. 
The air which has been climatically conditioned 
in this way now flows into the actual measuring 
section k, where its ice-forming capacity is ex- 
amined through the growth of a test object. 
Various measuring probes are also installed in the 
measuring section, and the test object can be il- 
luminated and observed at any time through 
plexiglass windows. In this connection it should 
be noted that the measuring section has in prin- 
