560 Prof. J. J. Thomson on the Masses of 



reason why the strong rays do not give good results is that 

 there are slight convection-currents in the vessel after the 

 expansion, for the walls of the vessel are warmer than the 

 gas ; this gives rise to convection-currents in the gas, the gas 

 going up the sides and down the middle of the vessel. The 

 velocity of the convection-current is added on to the velocity 

 of the ions due to gravity; and if the velocity of the ions is 

 very small, as it is when the rays are strong and the drops 

 numerous, a very small convection-current will be sufficient 

 to make the actual rate of fall of the drops very different 

 from that of a drop of the same size falling through air at 

 rest. All the reasons are operative in the case of ultra-violet 

 light, and it is only when the intensity of the light is small 

 that I have got consistent results. 



The vessel in which the expansion took place is shown in 

 fig. 2. AB is a glass tube about 3'6 cm. in diameter ; the 

 base CD is a quartz plate about "5 cm. thick ; on the top of 

 this there is a layer of water in electrical connexion with the 

 earth about 1 cm. in thickness ; the illuminated zinc plate was 

 3*2 cm. in diameter, and was 1*2 cm. above the surface of the 

 water. The ultra-violet light was produced by an arc about 

 '3 cm. long, between zinc terminals connected with an induc- 

 tion-coil; the arc was about 40 cm. below the lower face of 

 the quartz plate. The space between the zinc plate and the 

 water surface was illuminated by an arc-light so as to allow 

 the rate of fall of the drops to be accurately measured. The 

 tube LK connected this vessel with the apparatus used in the 

 previous experiments ; a figure of this is given in the Phil. 

 Mag. Dec. 1898. 



To observe the current of electricity through the gas, the 

 illuminated plate was connected with one pair of quadrants 

 of an electrometer, the other pair of quadrants being kept 

 connected with the earth. The capacity C of the system, con- 

 sisting of the plate, connecting wires and quadrants of the 

 electrometer, was determined. The plate was then charged 

 to a negative potential, and the deflexion of the electrometer- 

 needles observed. The induction-coil was now set in action, 

 and the ultra-violet light allowed to fall on the zinc plate : 

 the deflexion of the electrometer-needle immediately began 

 to decrease; the rate at which it decreased was determined 

 by measuring the diminution of the deflexion in 30 seconds. 



Let D be the original deflexion of the electrometer, let 

 this correspond to a potential-difference equal to «D between 

 the plate and the earth. If b is the distance between the 

 zinc plate and the surface of the water, the potential gradient 

 is aD/b. If A is the area of the plate, n the number of ions 



