ntact points being closed and opened during
half of each alternating current cycle.

Mechanical rectifiers are operated from the secondary windings of
transformers which reduce the voltage of the alternating current line
to the voltage desired for charging. Each rectifier unit may have its
own complete transformer, or one large transformer may operate a
number of rectifier units by having its secondary, or low tension
winding divided into a number of sections, each of which operates one
rectifier.

The advantages of the mechanical rectifier are its simplicity,
cheapness and portability. This rectifier also has the advantage of
opening the charging circuit when the alternating current supply
fails, and starting again automatically when the line is made alive
again. Any desired number of independent units, each having its own
charging line, may be used. The charging current generally has a
maximum value of 6 amperes. Each rectifier unit is generally designed
to charge only one or two six volt batteries at one time.

Stahl Rectifier

This is a unique rectifier, in which the alternating current is
rectified by being sent through a commutator which is rotated by a
small alternating current motor, similar to the way the alternating
current generated in the armature of a direct current generator is
rectified in the commutator of the machine. The Stahl rectifier
supplies the alternating current from a transformer instead of
generating it as is done in a direct current generator. Brushes which
bear on the commutator lead to the charging circuit.

The Stahl rectifier is suitable for the larger service stations. It
gives an interrupted direct current. It is simple in construction and
operation, and is free of delicate parts.

Other Charging Equipment

If there is no electric lighting in the shop, it will be necessary to
install a generator and a gas, gasoline, or steam engine, or a
waterwheel to drive it. A 10 battery belt driven generator may be used
in such a shop, and may also, of course, be used with a separate
motor. The generator should, of course, be a direct current machine.
The size of the generator will depend upon the average number of
batteries to be charged, and the amount of money available. Any of the
large electrical manufacturers or supply houses will give any
information necessary for the selection of the type and size of the
outfit required.

If an old automobile engine, and radiator, gas tank