Brush Discharge in Water and Salt Solutions. 465 



was connected to the outer case ; this was found necessary 

 in order to avoid electrostatic effects. 



The potential difference between the electrodes was mea- 

 sured by means of an electroscope in parallel with the cell ; 

 the electroscope being standardized by comparison with a 

 Kelvin electrostatic voltmeter. No change in either current 

 or potential difference was noticed when the platinum plate 

 was moved close up to the brush, which shows that the 

 potential difference measured was very little different from 

 the actual potential drop through the brush. 



No great accuracy in these measurements was possible, 

 however, owing to fluctuations in the electrical conditions. 

 These were steadiest with the uncondensed discharge, and 

 with the condensed discharge without a spark-gap. Readings 

 taken every five minutes varied 2 or 3 per cent, from the 

 mean value, but they were often constant for five or ten 

 minutes. Fluctuations in the condensed discharges were often 

 very large, the current dropping to a small value owing to 

 arcing occurring at the spark-gap, which needed constant 

 attention. 



It will be seen in the values given beside the photographs 

 (PI. V.), that while the current increases with the intensity 

 of the discharge the potential difference generally falls. 

 The fall in potential is of course only apparent as the sparks 

 through the brush become less frequent, as the discharge 

 becomes heavier. The fact noted, however, shows that the 

 resistance of the liquid (which resides chiefly in the brush) 

 is broken down most completely with the heaviest discharges. 



Character of the Discharge. 



The character of the discharge varies wdth the conditions 

 under which it is produced. The following Tables (pp. 466 

 & 468) summarise the main features. The reference to the 

 spectrum of each type of discharge is also given. Figs. 5, 6, 7 

 in text (p. 467) give representations of the various types of 

 discharge referred to in Table I. They are magnified about 

 twice. 



As regards the brush in distilled water, the current and 

 potential difference fall as the water becomes hot, owing to the 

 action of the discharge. Boiling water is almost kept boiling 

 by the discharge, and the brush in this case is very feeble. 

 The radical difference in the colour between the brush in 

 cold distilled water, and the brush in the same w r ater after it 

 has been boiled and allowed to cool, is rather remarkable. 



Phil. Mag. S. 6. Vol. 25. No. 148, April 1913. 2 K 



