124 ILLINOIS STATE ACADEMY OP SCIENCE 



zontal slit to confine the light to the region between the 

 plates. 



To observe the ray-tracks, compress the bulb B with 

 the hand until the water level reaches point A, Fig. 1. 

 Considerable pressure will have to be exerted on the 

 bulb to do this. At the same time depress the key K, 

 throwing the potential across the plates MM, NN. Then 

 release the bulb and the key at the same time, and if 

 everything is working properly, ray tracks should be 

 seen in the space between the plates. The function of 

 the electrostatic field is to sweep away the ions formed 

 by the impact of alpha-particles, except those that are 

 formed just as the expansion takes place and which 

 form the nuclei for the wisps or tails of cloud which are 

 the alpha-ray tracks. By repeated trials, the proper 

 amount of compression and the timing of the withdrawal 

 of the electrostatic field with reference to the start of 

 the expansion will be found which give the best results.. 

 With the field plates spaced about 1 cm, a potential of 

 110 volts across the plates has been found to give satis- 

 factory results. Five to eight blocks of 22.5 volt Eadio 

 B batteries may be used for this purpose, or the voltage 

 from a 110 volt DC supply circuit may be used. 



The ray tracks produced by this apparatus are, for 

 the most part, clear-cut and distinct. The unavoidable 

 turbulency of the air in the chamber, due to the presence 

 of the gauze plates and supporting rings, causes the 

 tracks to move slightly from their initial positions. How- 

 ever, this drift of the tracks after they have formed is 

 not serious when the extreme simplicity of the apparatus 

 producing them is kept in mind. Another point in the 

 operation of the apparatus is that after a dozen or so 

 expansions have been made, the under side of the top 

 surface of the chamber becomes clouded due to conden- 

 sation of water droplets upon it. However, this surface 

 may be cleared readily by tilting the apparatus from 

 time to time so as to wet the entire top end of the 

 chamber. 



