1896.] and on the zigzag path of Lightning. 221 



than others ; on these points my experiments do not enable me to 

 offer any opinion. 



Zigzag form of spark. When the path of a discharge is long, 

 the spark passes in the well-known zigzag course, often giving off 

 at the angles a series of side flashes or ramifications. This is 

 usually explained by supposing that " The spark finds this form 

 of path easier owing to the presence of irregularly distributed 

 particles of dust which act as stepping-stones, giving the discharge 

 a path of less resistance although longer." 



The action of the strips of metal in the previously described 

 experiments led me to think that the irregular form of the path 

 was due to particles charged by induction, attracting (not con- 

 ducting) the spark away from the straight line and the ramifica- 

 tions were discharges to these particles, after which the spark was 

 free to return to its natural course. 



In order to test the correctness of this idea I first fixed a 

 number of small pieces of tinfoil upon a piece of mica by means of 

 shellac varnish, and this upon an ebonite rod. When the stream 

 of electricity was passing between the poles of a Wimshurst 

 Machine, the mica disc was placed a little to one side of the path. 

 The sparks were drawn out towards the disc. 



In order to obtain the ramifications at the angles I placed a 

 large number of small brass beads upon silk threads about one 

 millimetre apart. These threads were then fixed to an ebonite 

 stand in parallel lines and as close together as could be managed 

 without allowing them to touch. When this was placed on one 

 side of a straight spark, the path immediately changed, forming 

 an angle towards the beads, and giving off the peculiar ramifica- 

 tions that are so often seen in photographs of lightning. 



The action of charged dust, &c. passing away from the nega- 

 tively charged conductor will no doubt produce part of this, 

 especially when the spark is short, but in the case of lightning I 

 believe the inductive action to be much more important. 



