154 Proceedings of the Royal Society of Edinburgh. [Sess. 
XVIII. — Experiments with an Electrified Pith Ball in an Ionised 
Atmosphere. By Dr Dawson Turner and Mr D. M. R. Crombie. 
(Bead May 2, 1921. MS. received June 16, 1921.) 
A pith ball suspended from the centre rod of a charged Leyden jar 
provides us with an interesting and very delicate method of demonstrating 
the ionised atmosphere surrounding flames and incandescent bodies. In 
an ionised atmosphere of sufficient intensity the ball, provided that the 
suspending fibre is of high insulating quality, rapidly loses its charge and 
falls back toward the centre rod of the jar, from which it receives a fresh 
charge and is violently repelled, only to repeat the performance; thus an 
oscillation is set up the rapidity of which is governed chiefly by the 
length of the suspending fibre. 
A screen of any material, even wire gauze, interposed between the 
ionising source and the ball, and not too near the latter, at once stops the 
oscillation. 
The violence of the oscillations seems from the experiments performed 
to depend upon the following factors : — 
(1) The distance of the source of ionisation. The nearer the source 
is to the ball, the more violent are the oscillations. 
(2) The nature of the source of ionisation. Thus the bulk of experiments 
showed that a bunsen burner was the most effective in producing oscilla- 
tions. Less effective sources included a naked Nernst filament, a glowing 
platinum wire, a candle flame, and a spirit-lamp flame. An electric arc 
gave comparatively poor results. This was somewhat surprising, but may 
be due to the powerful electric field between the carbons preventing the 
escape of ions to the surrounding atmosphere. To test if the effect had any 
relation to the actinic quality of the light, a piece of burning magnesium 
ribbon was tried, but was found to be less effective than a candle flame. 
Mere bulk of flame does not make much difference to the oscillations, 
since a jet of gas burning from an open pipe, though of large dimensions, 
was not so effective as the bunsen. 
(3) The third factor influencing the oscillations is the potential of 
the jar. Effects can be produced at a greater distance when the jar is 
highly charged than when it has only a small charge. This might seem 
to show the directive influence of the jar and ball in attracting a stream 
of the opposite kind of ions towards themselves. 
