PAPERS OX CHEMISTRY AND PHYSICS 121 



A SIMPLE FORM OF C. T. E. WILSON'S ALPHA- 

 RAY TRACK APPARATUS 



Chaeles T. Kxipp axd N. E. Sowers, University of 



Illinois 



About two years ago considerable interest was excited 

 among scientists by the appearance of the Shimizu modi- 

 fication of C. T. R. Wilson's clond apparatus for making 

 visible the tracks or traces of alpha particles thrown off 

 from radioactive material. The essentials of an alpha- 

 ray track apparatus are: (1) a closed chamber in which 

 alternate compressions and expansions of the air may 

 be made to take place, (2) means for setting up an elec- 

 trostatic field across the chamber, (3) a suitable device 

 for cutting off this field at the proper time with refer- 

 ence to the changes in volume inside the cylinder, and 

 (4) a source of alpha-rays inside the chamber. In the 

 Shimizu apparatus these essentials are elaborately and 

 efficiently provided for by (1) a cylinder of metal and 

 glass in which an airtight piston, whose length of stroke 

 is adjustable, is moYed up and down by an eccentric 

 crank on a shaft rotated by hand or by a small motor, 



(2) a conducting film of moist gelatine carrying some 

 CuS0 4 placed on the under side of the glass cover of the 

 chamber and a similar film of gelatine carrying India 

 ink placed on the top surface of the piston, between 

 which surfaces an electrical potential may be applied, 



(3) a suitably shaped, adjustable commutator attached 

 to and rotated by the crankshaft, so connected that it 

 serves to apply and withdraw the electrostatic field be- 

 tween the gelatine films at the proper times with refer- 

 ence to the compression and expansions inside the cyl- 

 inder, and (4) a trace of radioactive material carried on 

 the tip of a metal pin which projects into the chamber 

 through a ground-in, airtight bushing. 



It occurred to the writers that the alpha-ray tracks 

 might be effectively revealed by an exceedingly simple 

 apparatus built along the lines of a modified cloud ap- 

 paratus described by one of the authors a number of 

 years ago 1 , and which is at present being made by the 



1 Science III, Dec. 2 J, 1909, p. 930. 



