106 Dr. E. Weintraub on the Are in 
fact that the cross-section of the mercury arc is constant, 
while in the carbon are the section increases with the current. 
The independence of the voltage from the current is a 
property which all arcs have in common with the positive 
column of a Geissler discharge; and this suggests the idea 
that in the continuous passage of the phenomena observed 
in Geissler tubes to those classified under the name ares, 
the development of the positive column and the disappearance 
of the cathode space and negative column are the most pro- 
nounced factors. There is a potential drop at each of the 
electrodes which for two mercury electrodes gives a sum of 
134 volts, and for a mercury cathode and an iron or graphite 
anade a sum of about 74 volts. By inserting two platinum 
wires in the are and measuring the potential drop between 
each of the electrodes and the platinum wire, as well as that 
between the platinum wires themselves, one finds that the 
drop at the cathode amounts to about 5 volts, at the iron 
or graphite anode 24 volts, and at the mercury anode 
8 volts. These values differ a little from those given by 
Arons. The voltage drop in the arc outside of that of the 
electrodes themselves is exactly proportional to the length of 
the arc, and is smaller the larger the diameter. A simple 
relation between the potential ‘drop per unit length and the 
section of the tube does not seem to exist. The outside 
temperature being 15° to 20°, the potential drop in tubes of 
3-inch diameter is about 1°8 volt per inch. 
If the vacuum is perfect and the starting of the are made 
by means of an auxiliary arc, as described in § 1, the are 
presents from the very first moment a uniform appearance 
throughout all its length, except for a dark space above the 
cathode which afterwards disappears almost completely, and 
shows no striations whatever. 
Schuster found a similar result in case of a Geissler dis- 
charge through mercury vapour. In absence of air no 
striations could be obtained*. | 
The arc is at the first moment rather dim, and reaches its 
full brilliancy only after about 10 to 15 seconds (if the tube 
was cold to begin with). If, however, the vacuum is not 
perfect (especially i in absence of the carbon filament) so that 
the starting is slow, and the ionized vapour is seen to move 
slowly from the cathode to the anode, the are is usually 
preceded by the appearance of striations, starting from the 
anode and propagating toward the cathode. The differ- 
ence in appearance is intimately connected with a number 
of essential differences in regard to the voltage across the 
* Proc. Roy. Soc. vol. xxxvii. p. 318 (1884). 
