CONTEMPORARY ADVANCES IN PHYSICS 119 



of lithium bombarded by protons, has about eight milhon electron- 

 volts; if it enters an ionization-chamber filled with gas so dense that 

 it is brought completely to a stop, the ion-pairs appearing are about 

 a quarter of a million. The upper limit occurring in practice is 

 possibly twice as high, but is very rarely met with; there is no lower 

 limit, but every incentive to push downward and ever downward the 

 least amount of ionization which can be detected. 



Twenty-five years ago, it would have been impossible to detect by 

 electrical means so few as a quarter of a million ions. (The total 

 number produced e.g. by an alpha-particle was determined by meas- 

 uring the total ionization produced by a known and very great number 

 of particles.) This problem was however destined to be solved in 

 many ways, which I will group under four headings : 



(CI) By arranging to have each particle touch off a brief but violent 

 discharge, something like an invisible spark, in the gas of the ionization- 

 chamber. There is a strong electric field applied between the elec- 

 trodes of the chamber, whereby the "primary" ions which the particle 

 forms as it travels across the gas are caused to produce (directly and 

 indirectly) vast numbers of extra or "secondary" ions; and these 

 suffice to make a sensible effect in the external circuit. The idea was 

 first put into practice by Rutherford and Geiger in 1908, and the 

 scheme is commonly known by Geiger's name. One of the electrodes 

 must be either a fairly sharp point or a fairly thin wire, and there are 

 a number of empirical rules (some partially understood, some not at 

 all) about the size and shape of the chamber, the proportioning and 

 the conditioning of the electrodes, the nature and the purity and the 

 density of the gas, and the magnitude of the field. The voltage 

 across the gas must lie within a definite range, often pretty narrow; 

 if it is lower the particles do not produce discharges, if it is higher a 

 single discharge may last indefinitely. The ratio of the number of 

 secondary to the number of primary ions is usually not constant and 

 usually not measured; most of the various forms of the device serve 

 solely to detect or count the particles, and they are known as "Geiger 

 counters." Often a loudspeaker is connected into the circuit of the 

 ionization-chamber, and each discharge produces an audible clack, so 

 that by the Geiger method one hears the passage of a corpuscle as by 

 the Wilson method one sees it. Sometimes the discharges are recorded 

 and the record examined at leisure. 



(C2) By modifying the foregoing scheme so that the number of 

 secondary ions shall be proportional to the number of primary ions, 

 and a measurement of their total charge shall give at least a relative 

 value of the ionizing-power of the traversing particle. This is a 



