Metallic Vapours in an Exhausted Space.  —-: 1123 
interesting point is that under normal conditions no current 
flows between the two anodes E and C, even when they are 
only < inch apart. 
lf the tube is to work as a rectifier, the distances EB and 
BC are made small. If the arc is to be used as a source of 
light, the lamp must be long enough to consume all the 
voltage applied; and in that case the tube has the form 
similar to that of a direct-current lamp. First the lower part 
of the lamp is started, and as the ionized vapour fills the main 
vertical tube the arc is partly transferred to the two anodes 
at the top of the lamp; and this transfer can be made com- 
plete by opening the switches leading to the auxiliary anodes 
at the bottom of the lamp. As in the case of the direct- 
current arc, a carbon filament helps the propagation of the 
arc. The details, however, must be omitted, as they would 
lead too far into the practical side of the subject. 
In case of a three-phase alternating current a similar 
rectifier can be used. Three anodes, connected to the three 
wires of the system, are used. The three anodes are connected 
_by reactances to the mass of mercury at the bottom of the 
lamp, which is to serve as the common cathode. In the wire 
leading from the common cathode to the three reactances 
both half-waves of all three phases are rectified and superposed. 
The practical importance of the rectifier for alternating 
current is so self-evident that it need not be emphasized. It 
would, however, lead too far to enter into a description of the 
most practical form of such rectifiers, their use for large 
currents and for high voltages, the operating of many of them 
in multiple and in series, &c. The description given above 
was only intended to make known the main principle, and to 
point out those parts which are of scientific interest. 
Summary of Main Results. 
The main results of this investigation may be summarized 
as follows :— 
1. By a series of experiments it was shown that in the 
process of starting an arc the cathode plays an important rdle, 
so that a certain change must take place on its surface before 
the arc can start. The anode receives the current without any 
previous excitation. 
2. Starting from the recognition of this rdle of the cathode, 
a new method has been devised for an instantaneous starting 
of the passage of a moderate voltage-current through the space 
separating the electrodes, and this no matter how long that 
space is. 
3. The properties of the mercury arc have been studied, 
