The reaction of discharge may be explained thus: KOH readily splits by into the two "ions"' OH and K; the former tending to travel to the negative plate, and there combine to form Fe30. and H.0; while the K ion tends to travel to the positive plate, and there com bine with part of the oxygen of the NiO, and with water from the electrolyte, thus forming again KOH, exactly equal to the initial quantity.
The reaction may be represented as follows: 3Fe+8KOH +6N It-will be seen that the net result is a trans fer of oxygen from the positive to the negative, accompanied by a concentration of caustic potash at the positive and a dilution at the negative; the total amount of caustic in solu tion remaining constant throughout. The re action of discharge is held in check, and the electricity is furnished to the electrodes much as in the lead cell, described more fully on a subsequent page. During the process of charge the reactions are exactly reversed, with metallic iron as the result at the negative plate and nickel peroxide at the positive.
The reaction on charging. however, does not i take place directly in proportion to the amount of current, with the result that the charge must always considerably exceed the discharge in order to restore the cell to its initial condition. Moreover, the active materials in discharge are never completely converted according to the reactions shown; there always remain large proportions of metallic iron and nickel peroxide, even in a discharged cell.
Description of Edison Caustic potash solution has but slight effect upon steel and none upon metallic nickel ; hence nickel plated sheet steel constitutes an ideal material for almost all the structural parts of an alkaline cell, and is used for positive and negative plates, as well as for retainer.
The positive plate, Fig. 2, consists of a number of thin-walled steel tubes placed side by side and united by a surrounding frame, compresses the contents which are thus ually built up till the tube becomes full, when it is pinched off and later assembled with others likewise of sheet steel, nickel plated. Each tube is made tip of a spirally wound, and double seamed, strip of thin sheet steel very finely perforated and filled with the active material, nickel peroxide. This material, however, hap pens to be an extremely poor electrical con ductor, and in order to give it the necessary conductivity the material within the tube is interspersed with extremely thin transverse discs of metallic nickel, about 80 of them per inch length of tube. The nickel is prepared in
the shape of extremely thin flakes, and a meas ured quantity of these is fed into the tube, alternately with a measured quantity of nickel hydroxide (which is later converted into nickel peroxide). After each double charge of flake and hydroxide a ramrod enters the tube and into a complete plate. The usual diameter of the tubes is about one-fourth inch, though lat terly a smaller size is also used to some extent. In assembling the plates right and left wound tubes are used alternately to eliminate twisting tendencies, and each tube is strengthened by a number of tight-fitting steel rings at frequent intervals.
The detail with which the manufacture of this plate has been worked out is extremely in teresting, but limitations of space preclude a full description.
The negative plate (Fig. 3) is somewhat similar in design, with the difference that the active material, in this case metallic sponge iron, initially introduced as oxide of iron, is con tained within small rectangular pockets or boxes of perforated sheet steel. The surface of the boxes is corrugated to give strength, but as the iron inside is a good conductor there is not required the mechanical pressure which necessi tates the cylindrical form of the positive tubes. The pockets are about one-eighth inch in thick ness and are locked by pressure into a surround ing frame of sheet steel very similar to that of the positive.
Positive and negative plates are assembled alternately, with small hard rubber insulating strips between; and the plates of each kind are united by a bolt extending across the whole group, through the holes at the top corner, as shown in Fig. 4. Hard rubber frames around necessary in order to exclude the carbonic acid of the atmosphere, which is readily absorbed by caustic solution, and which if allowed thus to enter into an alkaline cell exercises a very detrimental effect upon its operation. A check valve in the cover permits the egress of oxygen and hydrogen liberated during charge, whtle it the edges and bottom of the group insulate it effectively from the steel case which holds the electrolyte and retains the whole cell as a working unit.