conductor,
offers a very high resistance to a current tending to pass through the
tube from the mercury electrode to the other electrode, but offers a
very low resistance to a current tending to pass through the tube in
the opposite direction. Current passes from the metallic electrode to
the mercury electrode through an are of mercury vapor which is
established in the tube by tilting it so the mercury bridges the gap
between the mercury and an auxiliary electrode just for an instant.

The absence of moving parts to got out of order is an advantage
possessed by this rectifier over the motor-generator. The charging
current from the rectifier cannot, however, be reduced to as low a
value as with the motor-generator, and this is a disadvantage. This
rectifier is therefore more suitable for larger shops, especially
where electric truck and pleasure cars are charged.

Mechanical Rectifiers

Mechanical rectifiers have a vibrating armature which opens and closes
the charging circuit. The circuit is closed during one half of each
alternating current cycle, and open during the next half cycle. The
circuit is thus closed as long as the alternating current is flowing
in the proper direction to charge the battery, and is open as long as
the alternating current is flowing in the reverse direction. These
rectifiers therefore charge the battery during half the time the
battery is on charge, this also being the case in some of the are
rectifiers.

The desired action is secured by a combination of a permanent magnet
and an electromagnet which is connected to the alternating current
supply. During half of the alternating current cycle, the alternating
current flowing through the winding of the electromagnet magnetizes
the electromagnet so that it strengthens the magnetism of the
permanent magnet, thus causing the vibrator arm to be drawn against
the magnet. The vibrator arm carries a contact which touches a
stationary contact point when the arm is drawn against the magnet,
thus closing the charging circuit.

During the next half of the alternating current cycle, or wave, the
current through the electromagnet coil is reversed, and the magnetism
of the electromagnet then weakens the magnetism of the permanent
magnet, and the vibrator arm is drawn away from the magnet and the
charging circuit is thus opened. During the next half of the
alternating current cycle the vibrator arm is again drawn against the
magnet, and so on, the co