40 EFFECTS OF /3-RAYS OF DIFFERENT VELOCITIES 



upon the roof of the .ionization chamber, and could be slid back and 

 forth so that a beam of ^S-rays of any desired radius of curvature 

 entered the hole. When these rays passed through the ionization 

 chamber the air in it became ionized. The relative number of ions 

 formed by the various beams of homogeneous rays was measured by 

 the rate at which they conducted electricity between the wall of the 

 ionization chamber, charged to 120 volts, and a suitably insulated 

 wire, W, which projected into the chamber from one end. This 

 wire led to a quadrant electrometer arranged as described by Duane 

 for measuring ionization currents of this sort.^^ Several corrections 

 had to be applied to the measurements of ionization made in this 

 way. A considerable part of the ionization current was due to the 

 effects of 7-rays which passed through all parts of the lead roof of 

 the ionization chamber and to secondary j8-radiations set up by them 

 on striking the walls of the chamber. A smaller increment was due 

 to the leak of electricity across the insulation of the wire. To correct 

 for these the roof of the ionization chamber was replaced by a piece 

 of lead in which there was no hole and a measurement made which 

 expressed the ionization due to the 7-rays and the leak. This value 

 subtracted from the former measurement, gave an expression of the 

 amount of ionization due to the /3-rays alone. As the air within the 

 ionization chamber became warm from the heat generated in the 

 coil its density became less and as a result the number of ions formed 

 by a given beam of rays was reduced in proportion to the rise in the 

 absolute temperature. Care was taken to record the temperature 

 of the air within the solenoid at the time each measurement was 

 taken and to correct each reading to a standard temperature, for 

 which 27°C. was chosen. In addition a sKght correction was made 

 for the decay of the radium emanation in the course of the experiment. 

 By these procedures figures were obtained expressing in arbitrary 

 units the relative ionization produced by each of the beams of /3-rays 

 which had been measured physiologically, and under exactly the 

 same conditions. 



^3 Duane, W., Boston Med. and Surg. J., 1917, clxxvii, 787. 



