252 Prof. Rijke on some Properties of the Induced Current, 



the contrary, before separating tliem they may be left to stay 

 for a longer time, so as to allow them to assume a higher tem- 

 perature. I have not noticed that the striking-distance in the 

 one is greater than in the other case. When the platinum 

 remains for a long time in the flame, its surface becomes rough- 

 ened, it does not slide so easily, the duration of the rupture 

 increases, and hence the striking-distance diminishes. 



6* If platinum wires are taken so thin that they assume a 

 very high temperature in the flame, and if they are withdrawn 

 from the flame while still in contact, and then contact is rapidly 

 broken without allowing them time to cool perceptibly, an explo- 

 sive distance is always obtained equal to that which would have 

 been found if contact had been broken in air at the ordinary 

 temperature. Hence it follows that the increase in the striking- 

 distance is simply due to the presence of a body in combustion, 

 and not to the higher temperature of the electrodes. 



I thought it necessary also to study the action exerted by the 

 different parts of the flame. The results obtained are given in 

 the following Table. The flame was produced by the combustion 

 of coal-gas. The electrodes of the induced current were a pointed 

 conductor and a circular plate : — 



Medium in which the rupture of 

 the inducing circuit took place. 



Greatest 

 striking distance. 



Ratio between the 



striking distances 



obtained when the 



rupture takes place 



in the given medium 









and in air. 







millims. 



m 



1 









, 21-9 





Hottest part of the flame ... 





/ 23-3 1 



1 23*7 J 



1073 







235 





Luminous part of the flame. 





/ 59-2 \ 

 1497/ 



2-486 







54-45 









J76-1-1 



1 65-6 / 



3-235 







, 70-85 





Hence it is not the hottest part of the flame which exerts the 

 greatest influence; on the contrary, it is that where the tempe- 

 rature is lowest. 



