[Fig. 49 Tungar rectifier bulb]

All rectifiers using oil are operated on the principle that current
can pass through them in one direction only, due to the great
resistance offered to the flow of current in the opposite direction.
It is, of course, not necessary to use mercury vapor for the arc. Some
rectifiers operate on another principle. Examples of such rectifiers
are the Tungar made by the General Electric Co., and the Reetigon,
made by the Westinghouse Electric and Manufacturing Co. The Tungar
Rectifier is used extensively and will therefore be described in
detail.

The essential parts of a Tungar Rectifier are: A bulb, transformer,
reactance, and the enclosing case and equipment.

The bulb is the most important of these parts, since it does the
rectifying. It is a sort of check valve that permits current to flow
through the charging circuit in one direction only. In appearance the
bulb, see Figure 49, resembles somewhat an ordinary incandescent bulb.
In the bulb is a short tungsten filament wound in the form of a tight
spiral, and supported between two lead-in wires. Close to the filament
is a graphite disk which serves as one of the electrodes. Figure 50
shows the operating principle of the Tungar. "B" is the bulb,
containing the filament "F" and the graphite electrode "A." To serve
as a rectifier the bulb filament "F" must be heated, this being done
by the transformer "T." The battery is connected as shown, the
positive terminal directly to one side of the alternating current
supply, and the negative terminal to the graphite electrode "A."

To understand the action which takes place, assume an instant when
line wire C is positive. The current then flows through the battery,
through the rheostat and to the graphite electrode. The current then
flows through the are to the filament and to the negative side of the
line, as indicated by the arrows.

During the next half cycle when line wire D is positive, and C is
negative, current tends to flow through the bulb from the filament to
the graphite, but as the resistance offered to the flow of current in
this direction is very high, no current will flow through the bulb and
consequently none through the battery.

[Fig. 50 Illustration of Tungar "half-wave" rectifier]
[Fig. 51 Illustration of Tungar "full-wave" rectifier]

The rectifier shown in Figure 50 is a "half-wave" rectifier. That is,
only one-half of each alternating current wave passes through