Or Figures produced by Electric Action. 503 



terminal than at the negative ; but with the poles not too far 

 apart they stretched across from one to the other in the form 

 of confused and irregular lines of flow. 



When the plate was laid as before on a metal sheet and 

 wires from both positive and negative terminals brought down 

 on the film, a discharge produced the characteristic figures 

 under their respective wires. These were best defined only 

 when no spark discharge crossed between the wires on the 

 plate ; and there was in this case no branching out of the 

 positive and negative markings towards each other, the induc- 

 tive circuit sensibly completing itself through the metal sheet 

 under the plate. 



When the difference of potential was made sufficient to 

 produce spark-discharge between the terminals on the plate, 

 the resulting marking depended in several respects on the 

 presence or absence of a metal sheet under the plate. 



With no metal sheet, and the plate insulated on a block of 

 paraffin, the discharge took a fairly direct course between the 

 terminals C (Plate VII.)? vsith only slight crookedness, but 

 sometimes in a double line. On this plate the characteristic 

 palm-fronds of the negative and dark branchings of the posi- 

 tive pole are wanting, and the lighter tracery forms a rough 

 indication of confused lines of flow 7 between the poles. 



With a sheet of foil pasted on the back of the plate (leaving 

 a margin of about 2 centim. round the edge) and spark- 

 discharge between the terminals, there is exceedingly little, if 

 any, distinct tendency of the positive and negative markings 

 towards each other. The track of the main spark is very 

 crooked, meandering over the plate in a quite irregular way, 

 and making sometimes sharp distinct angles in its course. 

 From about half of its length from each end, but principally 

 from the terminals, branch off here and there the characteristic 

 positive or negative markings. D, E, and F (Plate VII.) are 

 examples, the latter being the result of half a dozen sparks 

 passed over a plate in succession from a pair of terminals 

 resting on it. 



When a strip only of foil was fixed on the back of the plate 

 so that its length crossed the line joining the poles at an angle 

 of about 45°, as at G (Plate VII. ), where the rectangle shows 

 the position of the foil, and about twenty sparks were passed, 

 they all took a similar S-shaped course, having been appa- 

 rently attracted out of the direct line to follow that of the foil 

 underneath the gL 



The meandering form and general appearance of these 

 sparks, when acted on inductively by the foil under the plates, 

 remind one very much of certain kinds of lightning-flashes, 



2 M 2 



