178 MM. Elster and Geitel on 



consequently always a considerable diminution of the electro- 

 motive force. 



The difference of the numbers in one and the same column 

 arises from the circumstance that for each new determination 

 the wire a b was brought into a somewhat different place at 

 the margin of the flame. 



An experiment which likewise proves the dependence of 

 the electromotive force on the state of incandescence of the 

 electrodes, but which is not so free from objection as the 

 above, is the following : — The electrode S (fig. 2 a) was re- 

 placed by a platinum pan, and the electromotive force E deter- 

 mined. Water was then introduced into the red-hot pan, 

 and, as soon as it boiled, the quantity E measured again. 

 When the whole of it was evaporated and the pan again red- 

 hot, the first experiment was repeated for a control. In this 

 the turn-plate W (fig. 1) was placed so that the platinum pan 

 was conducted-from to earth. A long series of experiments 

 constantly gave the same result, namely a considerable dimi- 

 nution of the electromotive force with diminution of the tem- 

 perature of the pan. For example, 



(1) With the pan red-hot E = 216 



„ „ at 100° ........ E = 152 



„ „ red-hot (control-experiment) . E = 213 



(2) „ „ red-hot E = 196 



„ at 100° E = 118 



„ „ red-hot (control-experiment) . E = 197 



These experiments, without the confirmation afforded by 

 the preceding experiment, did not appear to us definitive, 

 because an alteration of E might possibly be effected by the 

 evaporation of the water and by the wetting of the outside of 

 the pan. But from the former experiment it was already 

 evident that the electromotive force is the greater the greater 

 the difference of temperature between the electrode in the flame 

 and that in the air. 



§ 13. Thermoeledrical Behaviour of Wires within a Flame. 



In contradiction to the fundamental experiments adduced 

 in the last section stands the fact that, in spite of great differ- 

 ences of temperature, no thermoelectric excitation takes place 

 when both electrodes dip equally into the flame. With the 

 electrodes arranged as represented in fig. 2 c it is easy to place 

 the electrode B so that it does not glow while S is intensely 

 white-hot ; but in spite of this the electrical forces which arise 

 are very slight, as the following experiment shows: — 





