IN AN ELECTRIC FIELD. HENDERSON. 127 



brass, were 7.10 cm. in iiitci'uiil ;iiiil 10.') cm. in rxtcrnal 

 diametrr. Tho tj;uar(l-rings V aiul D were held parallel 

 8.00 em. from one another by three glass rods f. The glass 

 rods G supi)orted the whole on the plate of an air-pump, to 

 whieh the lower plate was electrically connected. The glass 

 rods F and the outside edges of the guard rings were coated 

 with paraffin to prevent electrical leakage, but no insulating 

 material was near the field between A and B. 



To prevent diffusion of recombined rest-atoms from above 

 or below the plates, cotton wadding was placed over C and 

 under D to close up the small space between the rim of the 

 guard-ring and the bell-jar. This was the only essential 

 modification of the previous vessel. The effect of the wadding 

 will be referred to later. 



In filling the bell-jar with the required gas the former 

 was exhausted to at least 1 or 2 mm. pressure and the gas 

 was then allowed to pass in through a tube of CaCl2 and a 

 tube of P2O5. The bell-jar was again exhausted and the gas 

 allowed to enter a second time. The radium preparation 

 and two small dishes containing P2O5 were always kept in 

 the bell-jar, being placed above A. 



In this condition the apparatus was allowed to stand 

 from 12 to 24 hours in order to allow sufficient emanation to 

 collect. It should be noted that the greatest quantity of 

 emanation present at any time was minutely small, very 

 much less than that employed by Wellisch. This small 

 quantity would not cause the potential-gradient near the 

 plates to be much greater than that in the body of the vessel. 

 Hence the potential gradient would not be very ununiform. 

 For the last 4}^ or 5 hours' exposure of the plates to the 

 emanation the desired potential difference was applied 

 between them. Above a potential difference of 450 volts, i. e. 

 above a potential gradient of 150 volts per cm., the 

 potentials were supplied by a Wimshurst machine run by 

 an electric motor. The values of the potentials obtained 



