1908] on Recent Researches in Radio-activity. 37 



]»ro(lucfc of thorium. Such results are only to be expected on the 

 aljove view, since the a particle is the only common product of these 

 elements. 



The determination of the true character of the a particle is one 

 of the most pressing- unsolved prol^lems in radio-activity, for a number 

 of important consequences follow from its solution. Unfortunately, 

 a direct experimental proof of its true character appears to be very 

 difficult unless a new method of attack is found. We have seen that 

 if the charge carried by the a particle could be experimentally deter- 

 mined, the actual value of m could be determined in terms of the 

 hydrogen atom, since the value of the charge carried by the latter is 

 known. This could be done if we could devise a method of detecting 

 the emission of a single a particle, and thus counting the number of 

 particles expelled from a known quantity of a radio-active sul)stauce, 

 for example, from radium. In considering a possible method of attack 

 of this question, the remarkable property of the a particles of pro- 

 ducing scintillations in zinc sulphide at once suggests itself. Apart 

 from the difficulty of counting the scintillations, it is very doubtful 

 whether more than a small fraction of the a particles which strike the 

 screen produce the scintillations. Viewed from the electrical side, a 

 simple calculation from the data at our disposal shows that the 

 ionisation produced in a gas by a single a particle should be detectable. 

 The electrometer or electroscope used for measurement would, how- 

 ever, require to be extremely sensitive, and under such conditions it 

 is known that small electrical disturbances are very difficult to avoid. 



In order to obtain a reasonably large effect, we require some 

 method of magnifying the ionisation produced by the a particle. In 

 conjunction with Dr. Hans (reiger, I have recently developed a 

 method whereby the electrical effect produced by the a particle can 

 be magnified several thousand times. From the work of Townsend 

 it is known that if a strong electric held acts on gas at low pressure, 

 any ions generated in the gas by an external agency are set in motion 

 by the electric field, and under the proper conditions produce fresh 

 ions by collision with the gas molecules. The negative ion is the 

 most effective ioniser in weak fields, but when the voltage is increased 

 near the point at which a discharge passes, the positive ion also pro- 

 duces fresh ions by collision. In the experimental arrangement the 

 a particle from the active matter is fired through a small opening 

 about 2 mm. in diameter, covered with a thin layer of mica, into a 

 cylinder 60 cm. long and 2*5 cm. in diameter, in which the gas 

 pressure is about o cm. of mercury. A thin insulated wire connected 

 to the electrometer is fixed centrally in the cylinder. If the outside 

 cylinder is charged negatively, for a difference of potential of about 

 1000 volts any ionisation produced in the cylinder is increased about 

 2000 times by collision. This can be simply illustrated by using the 

 y rays of radium as a source of ionisation. When a difference of 

 potential is applied to the cylinder, the ionisation produced by the y 



