CURRENT BY MEANS OF AN ELECTROLYTIC RECTIFIER.
Oftentimes the only source of electrical energy for experimental work is
the 110 volt alternating current supply. This may be reduced to a
voltage suitable for operating small battery motors, trains, lamps,
etc., by means of a "lamp bank" resistance or a step-down transformer.
Direct current is *necessary* however in order to recharge storage
batteries and to operate many other devices. The electrolytic rectifier
is a device for changing alternating current into direct current and
will be found satisfactory for this purpose, provided too much is not
demanded of it. It is fairly efficient if used only to rectify small
amounts of current. It is not efficient when large amounts of currents
are passed through it and quickly becomes very hot in such a case.
An electrolytic rectifier consists of an electrode of iron or lead and
one of aluminum immersed in a solution of sodium phosphate.
[Illustration: FIG. 49.—A Single Cell of Electrolytic Rectifier.]
An ordinary glass battery jar may be used to hold the solution,
preferably one measuring 5 x 7 inches. The electrodes are supported by a
wooden cover which also serves to prevent the solution from evaporating.
The cover may be circular in form and simply rest on the top of the jar
or may have a groove turned on the underside so that it fits the rim of
the jar snugly.
It is a wise precaution to thoroughly saturate the cover with paraffin
by immersing it in a molten bath of that material. If the cover is
allowed to remain in the molten paraffin until all bubbles have ceased
to rise, the paraffin will thoroughly permeate the wood and protect it
from the action of the Chemical solution used in the rectifier.
[Illustration: FIG. 50.—An Electrode cut out of Sheet Metal. The top is
bent over at right angles and drilled so that it can be mounted on the
underside of the cover.]
As stated above, one electrode may be made of iron or lead. The other
should be aluminum. The electrode may be cut out of sheet metal and made
in the form of a strip about one and one-half inches wide and six inches
long. The top of each electrode is bent over at right angles and bored
with a small drill and an 8-32 brass machine screw is passed through the
hole and through the cover into the bottom of the binding post mounted
on the top of the cover, thereby serving not only to fasten the
electrode securely in place to the underside of the cover, but also to
establish connection between the electrode and the binding post itself.
Electrodes which are cut out of sheet metal possess the disadvantage
that they are not quite as efficient and will not last as long as
electrodes which are cast.
Cast electrodes are much heavier and far more efficient in many other
ways. They cannot be easily made by the young experimenter, but may be
procured from any one of several firms dealing in supplies for
experimenters.
The right hand sketch in Figure 52 shows how the electrodes should
appear when they are mounted in position on the underside of the cover.
They are placed about two and one-half inches to three inches apart.
The solution is formed by dissolving sodium-phosphate in water until a
"saturated solution" is formed, that is, until the water will not
dissolve any more. Sodium-phosphate dissolves rather slowly and it will
be necessary to stir the solution and crush the lumps which form with a
stick or glass rod.
Fill the jars nearly to the top and then place the electrodes into
position.
[Illustration: FIG 51.—A Cast Electrode will last much longer than one
cut from Sheet Metal. Cast Electrodes like that above are on the market
and can be purchased very reasonably.]
The action of the electrolytic rectifier, in changing alternating
current into direct current, is interesting and peculiar. The rectifier
acts much like a valve which opens one way and closes the other.
If a battery is connected to the electrodes of a rectifier, the positive
pole of the battery being connected to the lead or iron electrode and
the negative of the battery to the aluminum electrode of the rectifier,
the current from the battery will flow through the rectifier and nothing
unusual will happen. If, however, the poles are reversed so that the
positive pole is connected to the aluminum electrode, oxygen gas will
form on the aluminum. The action of the oxygen gas is to combine with
the aluminum and form a coating of *aluminum oxide* all over the
electrode. Aluminum oxide is an insulator and it therefore quickly forms
an insulating coating which shields the electrode from the solution and
stops the passage of the current. This action is almost instantaneous.
[Illustration: FIG. 52.—A completed single Cell Rectifier. The right
hand sketch shows how the Electrodes are mounted on the underside of the
cover.]
If the rectifier is connected to an alternating current supply it will
act just like a valve permitting the current to flow in one direction
but stopping it whenever the aluminum electrode is *positive*. The
resulting current is therefore, under proper conditions, an intermittent
current flowing only in one direction.
An alternating current may be represented by a wavy line drawn above and
below a straight line. Every time that the wavy line crosses the
straight line it represents an alternation or a reversal of the
direction in which the current flows.
[Illustration: FIG. 53.—A Diagram showing how a Rectifier cuts off
one-half of the Alternating Current Wave and changes it into Pulsating
Direct Current.]
The direct current from an electrolytic rectifier working under proper
conditions is practically one-half of the alternating current wave and
may be represented by the series of waves marked by "C" in the lower
part of the illustration in Figure 53.
It will be noticed that these lines do not cross the straight line and
the current therefore does not reverse but flows in one direction only.
When a single cell of rectifier is used on the 110 volt current supply,
it should be placed in series with a lamp bank or a step-down
transformer so that the current is reduced to lower voltage.
Figure 54 shows how to connect a storage cell in series with a single
cell of rectifier and a lamp bank so that the storage cell may be
recharged from the alternating current. A step-down transformer will be
found more efficient and not quite so wasteful of current as a lamp
bank. The rectifier and the storage cell are simply connected in series
with the secondary of the step-down transformer in order to secure this
result.
[Illustration: FIG. 54.—Circuit showing how a Single Cell of Rectifier
should be connected in series with a Lamp Bank to Recharge a Storage
Cell. A is the Aluminum Plate and L the Lead or Iron Plate.]
[Illustration: FIG. 55.—Diagram showing the Difference in Current after
it has been passed through a Single Cell or Rectifier and after passing
through a Four-Cell Rectifier.]
The negative pole of the storage cell must always be connected to the
aluminum electrode of the rectifier.
The series of little curved lines at the bottom of Figure 53 which
represent the alternating current after it has been changed into direct
current by the action of the rectifier, have a space between each two,
showing that there are periods during which no current flows but that
the current is intermittent and made up of a number of short impulses.
One-half of the alternating current is therefore really wasted.
It is possible, by means of four cells of rectifier, to so connect them
that "both halves of the alternating current wave are utilized" and the
spaces are filled up as shown in the lower part of Figure 55.
[Illustration: FIG. 56.—Diagram showing how a Four-Cell Rectifier is
connected. The Alternating Current Source is connected to C and D. The
Direct Current is taken off at A and B. The Electrodes marked A, A, A, A
are the Aluminum Electrodes. L, L, L, L may be Lead or Iron.]
The current which would normally be in a reverse direction, and
therefore below the straight line, has been completely reversed so that
it flows in the same direction as that above the line.
Figure 56 shows how to connect four cells of rectifier in order to
secure this result.
An electrolytic rectifier composed of cells approximately 5 x 7 inches,
as described, will not efficiently handle a current of over two to two
and one-half amperes. A four cell rectifier will operate to the best
advantage on a voltage over 50, and for that reason it is always best to
use a lamp bank or step-down transformer in connection with it.
When the solution in a rectifier becomes hot it will not operate as
efficiently as when cold. The solution becomes exhausted after a certain
amount of usage and requires renewal. This condition will be indicated
by the failure of the rectifier to operate properly. The old solution
should then be thrown away and the jars and electrodes washed and
cleaned thoroughly before the new solution is poured in.
[Illustration: FIG. 57—A Complete Four-Cell Rectifier connected together
and Mounted in a Tray.]