4(;r. Professor Harold Albert Wilson [Feb. 12, 



burners, sparks can be passed from the tip of one flame to the tip of 

 the other. Tlie temperature of the flame is about 2000° C, so that 

 the density of tlie gases in it is about one-seventh of their density at 

 the ordinary temperature. The potential difference required to send 

 a spark through the flame is about the same as that required to send a 

 spark through an equal length of air at one seventh of ordinary 

 atmospheric pressure. It appears therefore that the ions do not make 

 it easier for a spark to pass. This is due to the fact that the current 

 in the spark is greater than the ions can carry, so that the potential 

 difference has to be enough to produce more ions, and so is the same 

 in the flame as in unionised air at the same density. 



To study the conductivity of flame, it is convenient to use a row 

 of small Bunsen flames placed so that they touch each other. I use 

 a row of flfty flames burning from quartz tubes 1 cm. apart. This 

 gives a flame 50 cm. long, and about 10 cm. high. The quartz 

 tubes insulate very well, so that a current can be passed along the 

 flame liorizontally from one end to the other. 



If two parallel platinum electrodes immersed in the flame are con- 

 nected to a galvanometer and battery, it is found that a measurable 

 current is (jbtained. The relation between the current (/') and the 

 difference of potential (V) between the electi'odes is given by the 

 equation V = A*- + B<r/» where A and B are constants and d denotes 

 the distance between the electrodes. If d is small, say one or two 

 millimetres, the term Brf/" is negligible (except when l is very small), 

 and we get Y = A/^. In this case the current is almost independent 

 of the distance between the electrodes. 



The reason for this peculiar relation between the current and 

 potential difference becomes apparent when the variation of the 

 potential along the flame from one electrode to the other is examined. 

 An electrometer is connected to two platinum wires, which are 

 immersed in the flame and can be moved along horizontally between 

 the electrodes. Each wire takes up the potential of the flame at the 

 point where the wire is situated, so the deflection of the electrometer 

 indicates the difference of potential between the two points where the 

 wires are put in. Suppose one wire is allowed to touch the positive 

 electrode and the other is gradually moved along the flame from the 

 positive to the negative electrode. It is found that in the space 

 between the electrodes there is a small uniform potential gradient, but 

 near each electrode there is a comparatively sudden drop in the 

 potential. The drop near the negative electrode is much larger than 

 the drop near the positive electrode. Thus, nearly all the electro- 

 motive force of the battery is used up close to the negative electrode. 

 This shows that nearly all the resistance offered by the flame to the 

 passage of the current is close to the negative electrode. The positive 

 ions in the flame move towards the negative electrode and the nega- 

 tive ions towards the positive electrode, in fact the current is carried 

 through the flame bv these two streams of ions. Hence, close to the 



