3&2 Mr. C. A. Sadler on 



R 2 the gold-leaf retained its zero position so long as no 

 ionization was produced in the chamber I. When a secon- 

 dary beam passing through A and B entered I a steady 

 movement of the gold-leaf from its zero position took place, 

 and on cutting off the secondary beam the new position of 

 the gold-leaf, as viewed in the microscope, served as a measure 

 of the ionization which had occurred in I. A chamber, con- 

 sisting of two tubes, one end of each being closed, the one 

 being made to slide within the other, was charged to a 

 potential of —240 volts. Into one end of this chamber 

 passed an insulated wire connected at the other end to the gold- 

 leaf of E 3 . This chamber served as an adjustable compen- 

 sator by means of which the normal ionization in I could be 

 approximately eliminated. The compensator when once 

 adjusted remained unaltered while a complete series of 

 readings were taken. 



In making measurements upon the corpuscular radiation 

 it became necessary at each reading to adjust the position of 

 the aluminium window attached to the inner sliding tube 

 of the ionization-chamber I relative to the fixed tertiary 

 radiator R 2 . These adjustments involved changes in the 

 electrical capacity of the system. It thus became necessary 

 to calibrate the capacity of the whole system — consisting of 

 the ionization-chamber I, the compensator C, the electroscope 

 E 3 , together with the capacities formed by the wires con- 

 necting the chambers I and C to the electroscope E 3 and the 

 surrounding earthed tubes — for each position of the aluminium 

 window relative to the radiator R 2 occurring in the course of 

 subsequent readings. 



The method adopted was one suggested to the author by 

 Dr. Barkla, and was carried out as follows. 



A tube containing 5 milligrammes of radium bromide was 

 placed in a tunnel drilled into a block of lead ; the mouth of 

 the tunnel was then placed opposite to the side wall of the 

 compensator C. The radium produced an ionization in C which 

 soon reached a steady state. The ionization-chamber I was 

 connected to earth. The gold-leaf of the electroscope E 3 was 

 first earthed and then insulated. Owing to the ionization 

 produced by the radium in C the gold-leaf at once began to 

 deflect and the time taken for it to move over thirty scale- 

 divisions was observed. This was repeated for each position 

 of the aluminium window relative to the radiator R 2 required 

 in the subsequent experiments. In the following table are 

 given relative values of the capacity obtained in such a 

 series of readings; the capacity when the surfaces were 3 cm. 

 apart being taken as unity. Plotting the value of the 



