432 Mr. H. A. Wilson : Determination of the Charge 



The expansion apparatus used was kindly lent to me by 

 Mr. 0. T. R. Wilson, and it was similar to those he has 

 described in the papers referred to above. The apparatus 

 was arranged so that the valve V, the opening of which pro- 

 duces the sudden expansion, could be pulled back suddenly 

 by means of an electromagnet (M). This enabled the valve 

 to be pulled away every time in exactly the same way. 



The space in the tube AB below the disks was filled with 

 water so that the air between the disks was thoroughly satu- 

 rated with moisture. This air was rendered " dust free " in 

 the usual way, by repeated expansions with intervals in 

 between to allow the clouds formed to settle. The apparatus 

 was then ready for a measurement of e. 



A Rontgen-ray bulb was worked near AB, so that the 

 rays passed between the disks. Then the battery circuit 

 through the magnet was closed and a sudden expansion so 

 produced. A cloud was thus formed between the disks, and 

 the time which its upper surface took in falling from the 

 upper disk to the lower disk was measured. This gave vi 

 the rate of fall without an electric field. The experiment 

 was then repeated, but immediately after the expansion the 

 disks were connected to the battery, and so v 2 , the rate of 

 fall in an electric field, was obtained. 



It was found that if the rays were kept on all the time 

 during an experiment, then very large values for the charge 

 on each droplet were obtained. A field of a few hundred 

 volts per centimetre was then sufficient to cause many of the 

 droplets to rise instead of falling. It soon became clear that 

 the fresh ions formed after the expansion attached themselves 

 to the droplets, so that the longer the rays were kept on after 

 the expansion the bigger the charge on the droplets became. 

 A switch S was therefore put in the primary circuit of the 

 induction-coil used to excite the Rontgen bulb and arranged 

 so that the armature of the magnet turned the switch, broke 

 the circuit, and so stopped the rays a small fraction of a 

 second before the expansion was produced. 



The disks C and D were also connected to a commutator 

 which first connected them together, and then on being 

 turned connected them to the large battery used to charge 

 them up. 



A narrow beam of light was passed between the disks C 

 and D to illuminate the cloud and enable its upper surface 

 to be observed. The falling of the cloud was watched 

 through a small hole on a level with the disks, and about 

 twenty centimetres away from them in a direction nearly 

 perpendicular to the beam of light. A second screen was 



