504 Prof. Thomson, On Induced Radio-activity. 



On Induced Radio-activity. By Professor J. J. Thomson. 

 [Read 3 March 1902.] 



The investigation was undertaken with the intention of seeing 

 whether the ' induced radio-activity' shown by a metal rod after 

 long continued negative electrification in the open air would occur 

 if the rod were placed in a closed vessel instead of outside in the 

 open air. The closed vessel was a zinc gasometer 102 cm. high 

 and 75 cm. in diameter ; the vessel was insulated and used as one 

 of the electrodes, the other electrode was a metal tube placed at 

 the axis of the cylindrical gasometer. A potential difference of 

 800 volts between the cylinder and this rod was produced and the 

 current between these electrodes was measured. This current 

 was ' saturated ' and was therefore a measure of the total ioniza- 

 tion in the gas in the vessel ; if the rod became radio-active the 

 ionization and therefore the current would increase. 



The current was measured in the morning, and the rod in the 

 vessel kept connected with the negative terminal of a Wimshurst 

 machine for 6 or 7 hours, when it was disconnected from the 

 machine and the current again measured ; if the gas in the vessel 

 were not exposed to Rontgen rays whilst the rod was negatively 

 electrified I was not able to detect any increase in the current 

 through the gas as the result of the long negative electrification : 

 if however the gas were exposed to Rontgen rays during the 

 negative electrification of the rod, then a well-marked increase in 

 the current took place — the increase being some 16 or 17 per 

 cent. : this increase was due to some alteration in the rod and not 

 to a change in the gas in the vessel, for if a rod similar to the 

 one which had been electrified but which had not itself been 

 electrified were substituted the current sank to its former value. 

 No increase took place in the current if the rod were positively 

 electrified. 



A number of experiments were made on the currents through 

 the vessel when the vessel was not exposed to rays and when the 

 rod was not electrified. Rods of different sizes and different 

 metals were tried — these all gave approximately the same current ; 

 if the rod were carefully wrapped round with dry filter paper the 

 current showed a decided increase, while if the filter paper were 

 damp the current was many times its value for the bare rod : the 

 current in this case is greatest when the negative ions move up 

 to the paper-covered rod — a large effect is also produced when 

 the paper is wetted with brine or alcohol, but a solution of H 2 2 

 produces by far the largest effect yet found. 



