1898.] on photographic plates by electric discharges. 523 



The figures Nos. 1 to G were taken in air at different pressures 

 as given below. 



Positive Negative 



No. Pressure. No. Pressure. 



1 1 atmosphere 5 1 atmosphere 



2 75 



3 - 5 „ 6 "5 „ 



4 "25 



The figures were taken with all other conditions the same but 

 the pressure varied. The photographic plate was laid on a piece 

 of ebonite and put under a bell-jar in which the air could be 

 exhausted or compressed ; a pointed metal rod just touched the 

 surface of the film and was connected to the outer coating of a 

 Leyden jar set on the table, the inner coating of which was 

 joined to one of the knobs of a Wimshurst machine; the distance 

 between the knobs was the same in all the figures. 



The marked change in the figures produced by a change in 

 the pressure of the air shows that the form of the figure depends 

 to a great extent on the form which the discharge takes in the 

 air near the film ; even when the discharging point touches the 

 film the condition of the air near the film is the principal factor 

 in determining the nature of the figure. 



With the intensity of discharge used the negative figure 

 shows little detail at atmospheric pressure, while the positive 

 shows its characteristic fine tapering lines. As the pressure 

 diminishes both positive and negative figures increase in size 

 and the negative breaks up into branching feathery lines. At 

 the lowest pressure given (\ atmos.) the lines in both the positive 

 and negative figures have broadened out until nearly all the plate 

 is affected (the negative figure for this pressure is not reproduced 

 but it was very similar in appearance to the positive). 



Figures were also taken for pressures greater than atmospheric, 

 and as the pressure increases the figures get smaller and smaller, 

 the positive still showing its fine tapering lines and the negative 

 becoming a circular spot in which no lines can be detected. 



Now when we do the experiments in a dark room and watch 

 the discharge carefully the form of the visible discharge is the 

 same, as far as can be judged by the eye, as the form of the 

 figures ; in the positive we see the sharp branching lines and in 

 the negative we see less distinctness of outline. As the pressure 

 is lowered the lines broaden out until at the lowest pressures we 

 see only a continuous flash all over the plate round the dis- 

 charging point. The photographic figures are the same, but 

 showing more detail than we can notice with the eye. In the 

 figures given above the intensity of the discharge is small ; it 

 is interesting to increase the intensity and produce larger figures. 



