118 Dr. EH. Weintraub on the Are in 
A report on the rectification of a three-phase current by 
means of a mercury arc in vacuum, from the experiments of 
Peter Cooper Hewitt, appeared in the ‘ Electrical World and 
Hngineer, January ibe 1903. 
The line of thought I followed out was as follows :—The 
experiments described in section 1 have shown the important 
role of the cathode in the process of starting an are between 
two electrodes, one of which is a metal, and this observation 
immediately gave me an answer to the question, why is it 
difficult with moderate voltages to run an arc between two 
metallic electrodes? According to the experiments described 
at that place the passage of the arc is only possible when the 
ionization process is established at the cathode. In the case 
of an alternating current this peculiar condition of the cathode 
must be produced at each reversal of direction of current at 
a different electrode. The electrodes being separated the 
ionization process at the cathode soon dies out, and since the 
other electrode, which is now to be the cathode, is not excited 
the are cannot but go out. 
Experiments have heen accordingly carried out in order 
to see whether an alternating current arc through mercury 
vapours shows a behaviour in accordance with the theory 
expounded above. The experiments were arranged in a way 
similar to those in section 1. 
Experiment 14.—The tube ABCD (PL. IIT. fig. 9) has four 
cups filled with mercury. A and B are connected to a source 
of direct current, C and D to a source of alternating current 
of moderate voltage (100-200 volts). The direct current 
arc AB is started by bringing the mercury surfaces in con- 
tact and separating them, and then the ionized vapour fills 
the whole length of the tube. The are between C and D 
does not start up precisely as was the case with the direct 
current in section 1. As soon, however, as the electrode C 
is brought into contact with RB an arc is established between 
C and D in such a direction that C is cathode. If the contact 
between C and B lasts only a moment a direct current 
ammeter shows by its deflexion the passage of a rectified 
current. The are ceases immediately after C and B are 
disconnected. 
If now, the direct current remaining between A and B, 
the alternating current is applied to B and C, so that the two 
arcs have a common electrode, the alternating current will 
pass continually in the form of an arc between B and C, 
since B is constantly kept active by the direct current. The 
arc BC is, however, intermittent and the alternating current 
perfectly rectified, so that only that half wave passes which 
