1920-21.] Experiments with an Electrified Pith Ball. 157 
in one or two important respects. Firstly, as was to be expected, the 
distance at which oscillations could be produced was very much greater ; 
secondly, the effect could be produced as a rule at a greater distance when 
the potential of the jar was low than when it was high ; also, as the 
distance increased the oscillations took longer to commence, during which 
period the ball was slowing dropping toward the centre rod. Eventually, 
when the extreme limit at which oscillations could be produced was being 
approached, an interval of two or three minutes elapsed before oscillations 
began. At distances too great for the production of oscillations a dropping 
of the ball could be observed each time the X-rays were turned on. The 
interposition of an iron screen 035 mm. thick stopped the effect at once. 
In all the previous cases the potential of the central rod of the jar 
was the active means whereby a stream of ions was drawn from the source 
of ionisation so as to pass through the ball, hence when, instead of a single 
hall, four balls were attached round the circumference of the rod, one in 
each quadrant and equidistant from one another, only the ball in the 
direct alignment was affected, but in the case of the X-rays all the balls 
were almost equally affected, being all equally immersed in a sea of ions. 
Similar results, hut at much shorter distances, were obtained by using 
the rays from a 50-mm. capsule of radium bromide. The interposition of 
the iron screen did not stop the effect. 
Summary of Conclusions. 
(1) A pith ball suspended from the centre rod of a charged Leyden jar 
will oscillate in an ionised atmosphere and can serve as a very delicate 
indicator of the electrical condition of its surroundings. 
(2) By its means the ionised atmosphere around flames can be detected 
at a considerable distance, and the relative intensities of various sources 
of ionisation can be compared. 
(3) The ions are concentrated along the line joining the centre rod of 
the charged jar and the source of ionisation, for unless the pith ball be 
in this line it will be unaffected, except in the case of X-rays and the 
rays from radio-active bodies. 
(4) The ions tend to be carried upwards by convection currents. 
(5) The ionisation of the atmosphere does not depend upon the 
luminous or actinic intensity of a flame. 
(6) The effect upon the electrified pith ball appears to be independent 
of the nature of its charge. 
( Issued separately December 13, 1921.) 
