46-i 



W. Duane — Range of the a- Rays. 



Art. XLIII. — On the 



Range of the 

 Duane.* 



a- Rays / by William 



Fig. 1. 



The researches of Madame Curie, of Bragg and Kleeman, 

 and of .Rutherford have shown that the a-rays abruptly lose 

 their powers of ionizing gases, of affecting a photographic 

 plate, and of producing phosphorescence after they have pene- 

 trated several centimeters of air or an equivalent thickness 

 of other substances. Further, Rutherford has found that near 

 the point where it loses these powers the a-particle still 

 possesses sixty per cent or more of its initial velocity. 



Several years ago I made some experiments to determine 

 whether the charge carried by the a-particle could be detected 

 beyond the limit of its ionizing power, or, possibly, off to one 

 side of its range. The results were negative. Recently 1 

 have taken up the research again at the laboratory of Madame 

 Curie of the University of Paris, with more and purer radium, 

 and with the additional purpose of investigating the power of 

 the a-rays to produce secondary rays, and the transformation 

 of the kinetic energy of the a-particles into heat. 



Figure 1 represents the arrangement of the apparatus. A 



is a cylindrical box of brass 3*8 cm 

 long and 3'3 cm in diameter. A round 

 hole (l*8 cm in diameter) in the bot- 

 tom of the box is covered with a 

 very thin sheet of mica, B. The 

 mica weighs only 2 milligrams 

 per square centimeter, and is rein- 

 forced on the inside by a grating 

 of fine wures. It is so thin that 

 the a-rays can pass through it eas- 

 ily, and strike the plate C, which 

 is connected to an electroscope or 

 electrometer and serves as an elec- 

 trode. In order that a magnetic 

 - ... , r-i T R field may be produced parallel to 



Connections : P, to pump ; f e P kte B an( ? tc \ the ™ ica Window, 



L, to electrometer or electro- the apparatus is placed between the 

 scope ; E, to earth ; B, to bat- poles of an electromagnet ; and in 

 tery ; R, radium. order to produce an electric field 



between the electrode and the window, the ring D, to which 



the mica is fastened, is insulated from the sides of the box by 



wax, and connected to the pole of a battery. 



The method of procedure follows : A very small quantity of 



radium chloride was dissolved in water and recrystallized twice 



* Abstract of notes presented to the French Academy of Sciences. 

 Rendus, 11th and 25th of May, 1908. 



Comptes 



