1909-10.] Observations on some Spark-Gap Phenomena. 225 
the perforation consequent on the exciting of the electrodes to spark 
potential. Thus if the electrodes are circular brass discs pressed flat 
against a sheet of cardboard, one on either side, the appearance of the 
brush is as shown in fig. v. : it lies entirely on the + side of the card 
(thus in this case lending colour to von Waltenhofen’s hydro-electric 
theory *), and spreads out from opposite one single point on the — to the 
whole opposing side of the + electrode. After it has been passing for 
some time, the card is found to have a well-marked black band alongside 
the 4- pole. If now a hole be made in the card (e.g. a pin-hole) at any 
point between the electrodes, but not midway between them, the brush 
will pass through this hole, and will therefore lie partly on one side, partly 
on the other side of the card. With all the specimens of card examined it 
was found that, irrespective of polarity, the longer part of the brush 
assumed a violet-white tint, while the shorter part was of a bright light- 
greenish colour. 
Although, however, the point of* concentration of the rays usually 
determines the point of perforation, the spark does not, except in simple 
cases, follow the path of the brush discharge. Fig. vi. illustrates a typical 
case of this, ABC being the path of the brush, ADC the approximate spark- 
path. The point of concentration, A, is common to both paths. In this 
case a hole made in the card between the electrodes would have no appreci- 
able effect on the path of the brush. 
Some remarkable “ dark-space ” phenomena can also be easily observed 
or photographed. Fig. vii. gives an example : the observer is looking from 
above on to the spark-gap below. The + side of the card for the whole 
distance between the electrodes is strongly illuminated : from the — (conical) 
electrode proceeds little more than a point of luminous discharge— between 
* Pogg. Ann., Bd. cxxviii. p. 589. 
