Metallic Vapours in an Exhausted Space. 119 
has the common electrode as cathode, the other one being per- 
jectly suppressed. The electrode C, the anode of the rectified 
current (in the case of a common cathode), can be made of 
graphite, iron, or any other suitable material, just as in the 
case of a direct current arc. The wave shape of the current 
and electromotive force has been taken by means of a wave- 
form indicator, founded on the principle of charging a con- 
denser to the potential-difference to be measured, and dis- 
charging it through a galvanometer. A full description of 
the apparatus can be found in the ‘ Electrical World and 
Engineer,’ 1901, p. 688. | 
Plate IV. shows the wave shape of the current through 
the alternating current arc, together with that of the elec- 
tromotive force across the arc. It shows first how com- 
plete the suppression of one half wave is, and second, if 
compared with the potential curve of the alternator, that the 
form of the potential wave has not changed at all during the 
time when no current is passing. 
The reason of the rectification lies in the dissymmetry of the 
arrangement, only one electrode of the alternating are being 
made active. If both half waves are to pass the arrangement 
must be made symmeirical by providing two direct currents, 
one on each side of the alternating arc. 
Experiment 15.—In the glass tube ABCD (fig. 9) the cups 
A and B are connected to one, C and D to another source of 
direct current. B and C are connected to the alternator. 
B and Care the cathodes of the two direct current ares. (The 
connexions are not shown in the fig.) Under these condi- 
tions, since both electrodes of the alternating arc are kept 
active, both waves of the alternating current pass through 
the arc, as is shown by Plate V. 
Returning to the simpler case described in Experiment 14 
it will be noticed from Plate IV. that the current is sup- 
pressed for exactly 4 period. This is, however, true only for 
short ares. As soon as the length of the alternating arc 
exceeds a certain value (about 6 ins.) a peculiar kind of “ lag ” 
appears between the electromotive force and the current, so 
that the current lasts less than one half period. Plate VI. 
illustrates the case of a tube in which the length of the 
alternating are was 154 ins., and the anode of the rectified 
current a piece of iron. It is seen that the current started 
in the are very abruptly and only after the electromotive 
force had reached a certain value. The rise in current is 
simultaneous with a sudden drop of the potential across the 
arc. Plate VII. refers to an alternating are 36 ins. long. 
The cause of this lag is now the subject of investigation. It 
