August 3, 1882] 



NA TURE 



tial is, of course, equivalent to an electromotive force 

 acting along the flame. The want of concord amongst 

 different observers, not only as to the cause of the 

 electrical properties of flame, but even as to what those 

 properties were, is most singular. Probably a great part 

 of it arises from the omission to notice one very im- 

 portant point, namely, the part played in these electrical 

 phenomena by the sheet or mantle of hot air surrounding 

 the flame externally. Almost all these observers have 

 used as their instrument of investigation either a coarse 

 gold-leaf electroscope, or else a galvanometer. The 

 want of sensitiveness and accuracy in the former instru- 

 ment when applied to small differences of potential, makes 

 the former unsuitable ; whilst the high resistance offered 

 by the flame itself to the passage of electricity renders the 

 use of the latter inadvisable. 



In beginning their investigation, therefore, Messrs. 

 Elster and Geitel bethought themselves of Sir W. Thom- 

 son's quadrant electrometer as admirably suited for deli- 

 cate and quantitative experiments of the kind in hand, 



and with this instrument they set themselves to investi- 

 gate the correctness or otherwise of that which previous 

 observers had announced. 



Their apparatus was set up as follows : — To keep the 

 needle of the electrometer at a constant charge, it was 

 connected with one pole of a Zambonrs dry pile con- 

 sisting of 2400 pairs of disks, the other pole being joined 

 to " earth." The electrometer thus arranged was very 

 sensitive, a difference of potential of a single Daniell's 

 cell producing a deflexion of 112 degrees upon the scale 

 of the instrument, une pair of quadrants was as usual 

 put to "earth," and a suitable commutator (c in Fig. 1) 

 was interposed between the electrometer and the flame- 

 apparatus. Experiments were made on the flames of 

 Bunsen burners and of spirit-lamps, both well insulated. 

 A small Bunsen burner specially adapted for this purpose 

 was constructed out of a piece of glass tubing 4 milli- 

 metres wide. Fig. 1 shows the manner of exploring the 

 flame. One electrode, A, consisting of a platinum wire 

 inclosed in a glass tube from which the end protruded, 

 was inserted in the tip of the flame. Another electrode, 



ii, of similar kind was fixed so as to pass into the base of 

 the flame. Two points were revealed by the first experi- 

 ments made : 



1. That when set in this fashion the tip was usually 

 electrically negative, as compared with the base of the 

 flame, as Hankel had said, but that sometimes the reverse 

 appeared to be the case. 



2. That when the lower electrode was kept fixed, and 

 the upper one was moved to different heights, the poten- 

 tial anywhere within the interior of the flame was the 

 same, being 1-04 times that of one Daniell's cell when 

 the electrodes were 1 millimetre apart, and the same when 

 they were 20 millimetres apart. 



If, as Hankel expresses it, a flame were "polarised" 

 longitudinally, cross-sections taken horizontally across the 

 flame should be equipotential surfaces. This is true if 

 the two platinum electrodes a and B are both right within 

 the flame. Whether at the same level or not, when both 

 are completely within the flame they are practically at the 

 same potential — neither of them positive or negative rela- 

 tively to the other. But if one of the electrodes is dis- 

 placed to one side, a difference of potential is immediately 

 observed, and this difference is very great if (as in Fig. 1) 

 one electrode passes no further than into the external 

 mantle of hot air. 



To examine more particularly the part played by this 

 external mantic was the next point. It will be observed 

 from Fig. 1, that the two electrodes were so chosen that 

 the protruding portion of the platinum wire was equal in 

 length to the width of the flame from side to side. As 

 remarked above, when both were completely immersed 

 side by side in the flame, they showed no electrical dif- 

 ference ; but when either of them was moved into the 





surrounding sheet of hot air, it immediately showed itself 

 positive to the other one. The maximum difference of 

 potential was observed when the electrodes occupied the 

 relative positions shown in Fig. 2, a, where the electrode 

 B is about half a millimetre outside the flame. When B 

 was pushed in to the position shown in Fig. 2, b, the 

 potential fell to less than half what it was before. When 

 pushed completely in, as in Fig. 2, c, the two electrodes 

 were so nearly alike, that the difference of potential 

 between them was less than the hundredth part of that of 

 a single Daniell's cell. The upper electrode, A, was now 

 gradually removed. When it reached the position shown 

 in Fig. 2, d, it was decidedly positive relatively to B, and 

 when it was placed as in Fig. 2, c, its negative poten- 

 tial was almost as great as its positive potential had been 

 in the first position. The potential of A relatively to B is 

 given in the following table for the five positions : — 



a, E.M.F. was -1*29 of 1 Daniel). 



i. ,, -o"46 ,, 



c „ -o - ocq „ 



d. „ +0-65 ,, 



e. ,, H-fOQ ,, 



To put the matter into words : As long as either of the 

 electrodes is outside the flame and the other inside, the 

 outside one is positive and the inside one negative. The 

 film of hot air outside the flame is always electrically 

 positive, and the flame inside relatively negative. 



The same result was obtained by these experimenters 

 with flames of spirit-lamps and with ordinary gas and 

 candle flames. More curious still, when air was made to 



