Metallic Vapours in an Exhausted Space. 99 
the cathode, BA-mercury surface); on disconnecting B and A 
an are is formed which has A for a cathode, so that A is 
rendered active. 
The same results are obtained if in the same tube are AC 
is allowed to run, and the starting of BD attempted. The 
fact that the mercury are plays above the mercury in B 
does not make any essential difference. 
Instead of bringing the cathode C in contact with B, let 
us proceed as follows :— 
Experiment 3.—The connexions to BD remaining the same, 
let us apply the second source of current to the ter minals C and 
B of the tube, so that B is connected to the negative poles of 
the two eircuits. By some means, which will be clear from 
what follows later, the are BD is started ; if the switch sin the 
circuit of g’ is closed, the arc BC starts up instantaneously. 
The explanation of this experiment on the basis of expe- 
riment 2 is evident. 
Experiment 4.—Finally, let us dispense with the different 
circuits, and make the connexions in the way shown in 
fig. 2. In the tube ABD, A and B are two mercury elec- 
trodes and D is an electrode of mercury, graphite, or a 
metal like iron. (In the form shown by the figure the elec- 
trode D is supposed to be a piece of solid material, such as 
graphite or iron.) The negative pole of a source of moderate 
voltage, such as 125 volts, is connected to B; the positive is 
connected in parallel to Aand D. The length of the tube 
BD is limited only by the available voltage. AB is a short 
tube, about 1 inch long. ‘Two resistances are placed in the 
circuit: one, 7, in the positive line going to D, to limit the 
current in thearc; the other, 7; (75-150 ohms, with 125 volts 
impressed ), in the branch leading to A. If the surfaces of the 
two mercury electrodes, Band A, are brought into contact 
and separated again, a small arc is established between A and 
B, and the ionization process, being thus established on the 
cathode B, the ions propagate along the tube BD and an are 
starts between Band D. Jf the tube is well exhausted, the starting 
of the little are AB and that of the main are in the tube BD are 
simultaneous, and the whole process 1s nearly instantaneous. 
The arc AB can then be interrupted by opening a switch in 
the branch leading to A. A is called the “ auxiliary anode,” 
the tube AB the “ side branch,” or “starter.”” The operations 
described can be easily performed automatically by the current 
itself, and one of the many arrangements used is illustrated 
by figure 3. 
Experiment 5—The mercury fills the cups B and A 
(fig. 3) up to the level ef KL isa rod of iron, and § is 
H 2 
