614 Dr. H. Geiger and Mr. J. M. Nuttall on the Ranges 



— in the case of uranium — have recently been given by 

 Geiger and Rutherford*, and by Fochj. 



It would be of great importance if a method could be 

 devised which would be equally suitable for the accurate 

 determination of the ranges of all the known « ray products, 

 but at present no such method has been found. The main 

 difficulties result from the fact that some of the products are 

 gases, whilst some are very feebly active or only available in 

 the presence of other a ray products. 



In the present investigation we have employed a method 

 which appears specially suitable tor the determination of the 

 ranges of the substances whose activities are small. The 

 arrangement is indicated in fig. 1. The inside of a large 



Fig.]. 



glass bulb is silvered and connected to 

 a battery of about 700 volts. The active 

 film is placed in the centre of the bulb 

 on a small metal disk B which is con- 

 nected to the electrometer by means of 

 the wire H. The brass tube KK sur- 

 rounding the wire, and insulated from it 

 by ebonite plugs, serves as electrostatic 

 protection, and prevents any electrical 

 leak from the glass bulb to the wire. 

 The tube passes air-tight through the 

 ground-glass joint E, which fits into the 

 corresponding part F, sealed to the glass 

 bulb. The length of the wire H is care- 

 fully adjusted so that the plate B is 

 exactly in the centre of the bulb. If 

 the pressure in the bulb is reduced the 

 ionization produced by the a particles 

 will remain practically constant so long- 

 as the range of the u particles at the 

 particular pressure does not exceed the 



radius of the bulb, viz. 7 95 cm. But as soon as the pressure 

 is decreased below that value the ionization current will also 

 decrease. From this critical pressure and the radius of the 

 bulb the range of the a particles can easily be deduced. It 

 adds greatly to the accuracy of the determination of the 

 critical pressure if the active layer is very thin, and if the 

 area over which it is spread is small. Experiments with 

 polonium showed that plates with diameters up to 2'6 cm. 

 could be used without introducing an appreciable error. 



* H. Geiger and E. Rutherford, Le Radium, vii. p. 225 (1910), and 

 Phil. Mag. xx. p. 691 (1910). - ■ 



t A. Foch, Le Radium, viii. p. 101 (1911). 



