Metallic Vapours in an Exhausted Space. 121 
Experiment 17.—The tube differs from the previous: one 
in that there are two anodes in the main lamp. T (fig. 11) 
represents a transformer whose ratio of transformation can 
be supposed to be 1:1. The primary of the transformer 
is connected to the source; the two ends of the secondary 
are connected to the two anodes A and D, and a neutral 
wire, taken out from the middle of the transformer, is con- 
nected to the cathode B. By following out the arrows indi- 
cating the direction of the current, it is easily seen that each 
half-wave is in one of the two windings of the secondary, 
in the proper direction, and can pass through the arc, and 
that all the current that goes through the are is rectified and 
collected in the neutral wire of the transformer. The 
currents that actually flow in the two windings of the 
secondary, being of opposite directions, the saturation of 
iron which would take place with pulsating unidirectional 
current is prevented. 
Since current is constantly flowing through the arc, the 
direct-current arc in the side-branch fulfils only the function 
of keeping the cathode active during the short interval of 
time around the zero of the current in the main arc. 
1 have used many other forms, especially in connexion 
with polyphased currents ; but these experiments may be 
omitted here, as they introduce no new principle. | 
Although in experiment 17 the direct current in the side- 
branch could, even in case of single-phase alternating current, 
be reduced to only a small part of the rectified one, still the 
necessity for using a direct-current source is a disadvantage. 
Experiment 18 contains, however, the germ of an idea which, 
developed further, allows one to dispense with the use of the 
direct current altogether. 
Experiment 18.—The glass tube ABC (fig. 12) has three 
cups. The alternator is connected to A and C. A and B 
are connected outside by a reactance (of the magnitude of 
40 ohms). If A, B, and © are brought into contact and 
separated, the are runs steadily between A and B, and 
C and B, in such a way as to have B for a common 
cathode, and A, © for two anodes. The explanation of 
the action of the reactance placed between A and C is 
as follows:—Let us assume that the alternating current at 
a certain moment has the direction CB. It can then go 
through the tube in that direction, making B cathode, and 
assing through the reactance which is in series with CB. 
When this halt-wave begins to decrease, the reactance restores 
the energy stored up in it by discharging itself through AB, 
this discharge having B again for the cathode. Before that 
