Brush Discharge in Weak Acids and Solutions. 808 



Platinum wire about *5 mm. in diameter was used in the 

 construction of the points, one -point lasting usually for two 

 exposures. Copper wire of about the same diameter was 

 used in a few trial experiments, but was not satisfactory. 

 For the negative brush it acted as well as platinum, but the 

 positive brush caused the glass to crumble rapidly away. 



Features of the Brush. 



The general features of the brush when it is not unipolar 

 have already been described *. Differences occur according 

 to the sign of the brush which call for further description. 

 In distilled water each brush consists of a bundle of very 

 fine filaments (fig. 2). Unless the point has been filed 



Fio-. 2. 



Brush (+ or -) in distilled water. 

 [Figs. 2 to 5 represent brush enlarged about twice.] 



very carefully the brush in either case may split in two, 

 each brush springing from a point on the edge of the 

 platinum wire where it touches the glass. The positive 

 brush is shorter than the negative and emits a brilliant rose- 

 red light. The greatest length attained by the negative 

 brush is about 5 mm. It emits a pale blue light, almost 

 white in its intensity. A slight rise in conductivity of the 

 water causes the colour to change to red, though it never 

 attains to the bright rose-red of the positive brush. As the 

 conductivity rises (the increase due to the rise in temperature 

 produced by the action of the brush is sufficient), the fila- 

 ments of the positive brush thicken, diminish considerably 

 in number, and emit a somewhat feebler light. Under the 

 same circumstance the negative brush shrinks, grows fainter, 

 and at length emits a feeble blue light with yellow close to 

 the point. If the current in the transformer primary is kept 

 constant, these changes are accompanied by an increase of 



* Smith, /. o. 



