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 7 5 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 
w r as ‘ 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 0 2 
produces by far the largest effect yet found. 
