174 T. S. Taylor — Retardation of " Alpha RaysP 



of the sheet of gold from the polonium, corresponding to these 

 deflections as ordinates (the polonium being kept at a distance 

 of 3'0 cm from the ionization chamber). Curves similar to 

 III and IV in figure 3 were also plotted for the gold foil when 

 the polonium was set at distances other than 03 ,cm from 

 the ionization chamber. The distance of the polonium from 

 the ionization chamber was always so chosen that the chamber 

 cut the Bragg curve somewhere in the slightly inclined por- 

 tion. The Bragg curves I and II were determined each time 

 before making the measurements plotted in curves III and IV 

 for any given distance between the polonium and the ioniza- 

 tion chamber. In this way, several sets of curves similar to 

 the ones in figure 3 were obtained for each metal sheet given 

 in Table II. 



The diminution in the range of the a particle produced by 

 the sheet when 6? 1111 from the polonium (its air equivalent in 

 this position) was determined by first plotting the ionization 

 curve without the sheet over the polonium and then with the 

 sheet over it. The difference between the ordinates of the 

 two curves corresponding to a given abscissa, which was the 

 deflection of the electrometer needle, was the diminution in 

 range of the a particle produced by the sheet, or what is the 

 same, its air equivalent. This is the same as taking the deflec- 

 tion of the needle without the sheet over the polonium when 

 it is at a certain distance from the chamber, and, after placing 

 the sheet over the polonium, noting how far the polonium 

 and sheet must be moved towards the ionization chamber to 

 get the same deflection of the needle. 



Determination of the 'Variation in the Air Equivalents. 



As previously stated, when the polonium, with a metal sheet 

 6 mm above it, was set at such a distance from the ionization 

 chamber that the chamber cut some part of the slightly inclined 

 portion of the Bragg curve, it was found that, by moving the 

 sheet farther away from the polonium, the ionization increased 

 as the distance of the sheet from the polonium increased. This 

 signifies that by moving the sheet away from the polonium 

 the entire ionization curve was pushed upward, so that the 

 part of the slightly inclined portion, which fell just below the 

 ionization chamber when the sheet was 6 mm from the polonium, 

 fell within the chamber when the sheet was at a little greater 

 distance from the polonium. 



This means that the total range of the a particles is greater 

 with the sheet at the greater distance from the polonium ; and 

 the magnitude of this increase in total range (which is also the 

 diminution in the air equivalent of the sheet) can be obtained 

 directly from the measured ionizations, given in curves. Ill and 

 IV, and the curves I and II, fig. 3. 



