CHAPTER XV.

GALVANIC CELLS AND BATTERIES.

=EXPERIMENT 108. To study the effect of dilute sulphuric acid upon carbon and various metals.=

_Apparatus._ A piece each of copper and iron wire 4 in., (10 cm.) long; two narrow strips of sheet zinc (No. 60), one being amalgamated (§ 257); a carbon rod (No. 64); a tumbler (No. 65), partly filled with dilute sulphuric acid, (§ 258); mercury (No. 52).

=257. To amalgamate= one of the above zinc strips is to coat it with mercury. Remove all jewelry from the hands before proceeding. Wash the zinc with water, and with a cloth remove all dirt from its surface. Amalgamated zinc is very brittle, so lay it flat upon a piece of board or upon a plate, after dipping it for an instant in the dilute acid. Place a small drop or two of mercury upon the strip, and rub the mercury about upon both sides of the zinc with a cloth made wet with the dilute acid.

Mercury clings strongly to zinc or tin, so you may use a narrow piece of either as a spoon to carry a small drop from the supply to the zinc. Tap it upon the zinc to dislodge the drops. Do not get on too much mercury, just enough to coat it, or, at least, that part of it that will be under the acid. Be careful not to break the thin zinc when amalgamating it, as it gets very brittle. It should look bright. (See Apparatus Book § 32, 33.)

_Note._ Should any mercury get upon copper plates it may be removed by heating them in a flame.

=258. Dilute sulphuric acid=, for these experiments, should be made by mixing 1 part, by volume, of concentrated acid, with 20 parts of water. Do not let the acid get upon clothes or carpets. Do not add water to acid. Mix by _slowly_ adding the acid to the water in a glass or earthen dish, stirring at the same time. Mix over a sink or out of doors. (For fuller details see App. Book; § 21, 22, 23, 24, 25.) To save time, make at least a quart of the mixture, bottle, and label it for use.

=259. Directions.= (A) Bend over one end of each of the wires and metal strips, and hang them upon the edge of the tumbler (Fig. 81), so that their lower ends shall be in the acid. Do not let them touch each other. Stand the carbon rod in the acid.

If there is no visible action upon any of the above substances, add a few drops of concentrated acid to the tumbler.

Note carefully what takes place in the tumbler, and state which of the substances are dissolved, which simply made brighter, and which not acted upon at all.

_=260. Discussion.=_ The bubbles of gas that arise from the zinc when it dissolves are hydrogen, and they indicate that a vigorous chemical action is going on in the tumbler, and that the zinc is being eaten away.

=EXPERIMENT 109. To study the effect of dilute sulphuric acid upon various combinations of metals.=

_Apparatus._ The same as in the last experiment. A small piece of amalgamated zinc, however, will be better than the whole strip.

=261. Directions.= (A) Twist one end of the clean copper wire around the small piece of amalgamated zinc (Fig. 82). Hold one end of the wire in the hand and dip the combination into the acid. What takes place? Watch the surface of the copper, remembering that each, alone, was not acted upon by the acid (Exp. 108).

(B) Use the clean iron wire in place of the copper wire, and repeat (A). Watch the surface of the iron.

(C) With a string or thread tie a small piece of well amalgamated zinc to the carbon rod (Fig. 82), then dip the combination into the acid. Watch the surface of the carbon.

_=262. Discussion.=_ While amalgamated zinc is not rapidly dissolved by dilute sulphuric acid, a vigorous action of some kind takes place when it is in contact with another metal or with carbon in the acid. The bubbles of hydrogen that are liberated do not seem to come from the zinc; they appear to grow, in the fluid, directly at the surface of the copper, iron, or other metal used with the zinc. This shows that something besides the mere dissolving of a metal takes place.

Can we arrange our apparatus so that we can get some useful results from this action?

=EXPERIMENT 110. To study the construction of a simple Voltaic or Galvanic cell.=

_Apparatus._ A narrow strip of zinc (No. 60), amalgamated as directed in § 257. (An amalgamated zinc rod (No. 74) may be used in place of the strip); a narrow strip of sheet copper (No. 67); the tumbler of dilute acid of Exp. 108; a flexible copper wire about 2 feet long, with spring connectors (No. 54) attached to its ends. (See Electrical Connections, § 226.)

=263. Directions.= (A) Holding the amalgamated zinc strip in one hand and the copper strip in the other (Fig. 83), dip them into the acid, but do not let them touch each other. Note any chemical action.

(B) Touch the copper and zinc together, _below_ the surface of the acid. Watch the copper.

(C) Separate the lower ends of the strips, then touch them together _above_ the acid. Anything still happen to the copper?

(D) Slip one spring connector with the attached wire upon the zinc strip, then stand the strips in the tumbler, so that they can not touch each other. Now touch the copper strip with the free end of the wire, at the same time watching the copper.

(E) Raise the wire from Cu, touch it to Cu again, and repeat several times until you are sure that something takes place every time the wire touches Cu.

_=264. The Electric Current.=_ Something must happen in or through the wire, and it can only happen when the two metals are joined in some way. This peculiar, invisible action in the wire is called the _electric current_, and such an arrangement is called a _Galvanic cell_.

_=265. Source of the Electrification.=_ When two different metals are placed in acid they are electrified unequally by chemical action. Each has a higher potential than the acid, but their potentials are not the same. This electrification tends to pass from the place of higher to the place of lower potential, and the conducting wire allows this transfer to take place. As the difference of potential is kept up by the continued chemical action, the current is continuous, and not simply instantaneous, as in discharges of frictional electricity. As heat is produced by the burning of coal, so electrification is produced by the chemical burning of zinc. Chemical energy is the source of electrification in the Galvanic cell, just as muscular energy was the source of the electrification in the experiments with frictional electricity.

_=266. The Electric Circuit; Open and Closed Circuits.=_ The simple Galvanic cell just used, together with the wire which joined the metal strips, is called an _electric circuit_. If the current should pass through a telegraph instrument, for example, on its way from one strip to the other, the telegraph instrument would also be in the circuit. When the wire is cut or removed from one metal strip, the circuit is said to be _open_--that is, we have an _open circuit_. When the current passes, the circuit is _closed_. We also say _make_ and _break_ the circuit, and that the circuit has been _broken_.

_=267. Plates or Elements.=_ The copper and zinc strips are called the _plates or elements_ of the cell. The zinc, Zn, Fig. 84, is dissolved by the acid, and is called the positive plate (+ plate). The copper, Cu, is the negative plate (- plate). The copper is also called the _cathode_, and the zinc the _anode_.

_=268. Direction of Current.=_ It has been agreed, for convenience, that _in_ the cell the current passes from the zinc through the liquid to the copper, where the hydrogen bubbles are deposited. It then passes through the wire, or other conductor furnished, back to the zinc, through the liquid to Cu again, and so on around and around thousands of times per second. The current really starts at the surface of the zinc, where the chemical action is. When carbon and zinc are used, the action and direction of the current are the same, carbon being the - plate.

_=269. Poles or Electrodes.=_ If the wire were cut, the electricity coming from the + plate would be stopped at the end of the wire marked +, Fig. 84, after passing through the acid and up Cu. This end of the wire is called the + _pole or electrode_ (positive). The end of the wire joined to Zn is called the - _pole_ or _electrode_; that is, the + electrode is the end of the wire attached to the - plate. The tops of Cu and Zn may be considered electrodes. The top of Cu is the + _pole_ of the cell, while Cu is the - _plate_.

=270. Chemical Action in the Simple Galvanic Cell.= The chemical formula of sulphuric acid is H_{2}SO_{4} (read H, 2, S, O, 4). This means that it is a compound of hydrogen (H), sulphur (S), and oxygen (O). The H_{2}SO_{4} stands for _molecule_ of acid, and the small figures show that the molecule is made up of 2 _atoms_ of hydrogen (H_{2}), one of sulphur (S), and 4 of oxygen (O_{4}). The atoms are held together by _chemical attraction_ or _affinity_.

There is a stronger chemical affinity between zinc (Zn) and SO_{4} than between H_{2} and SO_{4}; so, as soon as the Zn gets a chance, as it does in the cell, it drives out the H_{2}, and it takes its place in the molecule. This chemical _reaction_ may be shown by the following chemical _equation_:

Zn + H_{2}SO_4 = ZnSO_4 + H_2.

Zinc and sulphuric acid produce zinc sulphate and hydrogen.

The zinc sulphate produced weakens the effect of the acid; in fact, the acid has to be renewed occasionally, as it is changed to the sulphate which remains in solution.

=271. Action in Cell Using Impure Zinc.= The above action takes place in the cell when impure zinc is used, even when no current passes, heat being produced by the reaction instead of useful electricity. (See Local Currents.)

=Action in Cell Using Pure Zinc.= When pure zinc (or impure zinc that has been properly amalgamated) is used in the cell, it dissolves, or is eaten away, only when the current passes. It should be noted that the bubbles of hydrogen do not even then appear at the zinc; they are not seen throughout the body of the liquid. There seems to be an unseen transfer of hydrogen from the zinc, through the liquid, to the copper (or other - plate used), and it appears there in the shape of bubbles. The larger the quantity of pure zinc dissolved, the stronger the current, and the larger the amount of hydrogen produced.

As the zinc dissolves it parts with its latent energy, and this energy forces the electric current through the circuit. While the hydrogen of the decomposed acid makes its way towards the - plate, the SO_4 part of it travels towards the Zn plate, where the ZNSO_4 is formed. (See § 270.)

=EXPERIMENT 111. To see what is meant by "local currents" in the cell.=

_Apparatus._ Tumbler of dilute sulphuric acid. (§ 258); strip of unamalgamated zinc; crystal of copper sulphate (blue vitriol) (No. 86); a galvanized iron nail (No. 69), this being iron covered with zinc.

=272. Directions.= (A) Hold the nail in the acid for a few seconds, and note result.

(B) Rub the copper sulphate upon the zinc simply in one spot, then place the zinc in the acid, noting the result at the spot.

_=273. Local action; Local Currents.=_ Ordinary commercial zinc contains such impurities as carbon, iron, lead, etc., in small quantities. It was seen, Exp. 109, that when different metals were in contact with the zinc, the zinc was rapidly dissolved by the acid. The impurities in the zinc act like the copper plate in the simple cell, thus producing _local currents_ in the zinc, which rapidly destroy it without doing any good. These currents pass from zinc to impurities, and back to the zinc, without going out into the main wire. This local action takes place even when the main circuit is open.

_=274. Reasons for Amalgamating Zinc Plates.=_ Pure zinc is not affected by dilute sulphuric acid, but it is too expensive to use in cells; so amalgamated zinc is used instead, because it is cheaper, and acts the same as pure zinc. The impurities are removed from the surface of the zinc, as the zinc alone is dissolved by the mercury. There is, then, a liquid layer of pure zinc with mercury upon the surface of the amalgamated plate. This is not acted upon by the acid when the circuit is open. A stronger and more regular current is produced with amalgamated zinc than with the impure unamalgamated zinc.

=EXPERIMENT 112. To study the "single-fluid" Galvanic cell.=

_Apparatus._ The galvanoscope G V (No. 58), (See § 240, etc.); the simple cell arranged as described in § 275, the zinc being amalgamated.

=275. The Simple Cell= should be arranged so that the plates will be held firmly in position. The zinc, Zn (No. 60), and copper, Cu (No. 67), should be fastened to the wooden cross-piece, W C P (No. 70), as shown in Fig. 85. Care should be taken not to use longer screws than those provided for (No. 72). If the screws touch each other they will short circuit the cell. Partly fill the tumbler (No. 65) with dilute sulphuric acid (§ 258), join wires with connectors to the plates. The free ends of the wires are then ready to join to apparatus. The ends of wires _may_ be fastened under the screw-heads instead of using connectors on the plates. Do not put the plates into the acid until you read the "directions."

=276. Directions.= (A) Arrange as in Fig. 86. Place the coil of G V, N and S (§ 244). _Before_ putting the plates in the acid join them to the 15-turn coil of G V (§ 242). The compass-needle should point to zero. See that the needle swings freely.

(B) Place the plates in the acid, and _quickly_ bring the needle to rest with the aid of the hand, so that you can take the reading at once before the hydrogen bubbles entirely cover the copper plate. Watch the action of the needle for a few minutes. Make a note of the reading, in degrees, at the beginning of the experiment and at the end of five minutes. (See Note.)

=Note.= If no change takes place in the position of the needle, the change beginning inside of 10 seconds after the plates are let down into the acid, withdraw the plates, then clean and thoroughly dry the copper to remove all traces of hydrogen. This may be done by heating the copper over a gas flame. Let the copper remain in the air 15 minutes, then try again. In taking the first reading you must work quickly. Catch the needle during its _first_ swing. If you allow it to swing back and forth until it comes to rest, your first reading will not be what it should be.

(C) After the needle has remained in one position, without change, for 2 or 3 minutes, take hold of the wooden cross-piece, move the plates back and forth in the acid to dislodge the hydrogen bubbles, and note carefully the action of the needle. Does the current seem stronger when the plates are moved? Can you get the needle back to the first reading?

(D) Remove the plates from the acid, dry and clean the copper, let them stand in the air for 15 minutes, then take another quick reading and compare it with the first.

_=278. Polarization of Cells.=_ The acid gets a little weaker, of course, as it is decomposed by the zinc (§ 270), but the chief cause of the weakening of the current is hydrogen, which forms a filmy coating upon the copper plate. This coating even seems to soak into the copper, and it takes some time for it to be thoroughly removed. The zinc plate is kept comparatively free from hydrogen by amalgamation.

_=279. Effects of Polarization.=_ The hydrogen bubbles weaken the current in at least two ways. In the _first_ place, hydrogen is not a conductor of electricity; so it holds the current back, as any other resistance would.

_Secondly_, acid acts upon hydrogen as it would upon another metal. When the copper plate becomes well covered with hydrogen, the acid cannot touch it; so we really have a _hydrogen plate_ in the cell. Hydrogen acts very much like zinc in the acid; we say that it is more electro-positive than copper. The result is, then, that a new current starts up, and as this is towards the zinc, in the acid, it partially destroys or neutralizes the main zinc-to-copper current. Practical use is made of the principles of polarization (see Secondary Batteries).

_=280. Remedies for Polarization; Depolarizers.=_ Any scheme by which the hydrogen may be destroyed and kept from the inactive, or negative plate, will prevent polarization. _Mechanical_ means have been employed to brush away the hydrogen by keeping up a constant circulation of the liquid. _Chemical action_ is another means by which the hydrogen may be side-tracked before it gets to the - plate in single-fluid cells. Substances like nitric acid and bichromate of potash, called _depolarizers_, contain large quantities of oxygen, and, during the chemical changes that take place, this oxygen unites with the hydrogen. These substances are used in zinc-carbon cells. (See § 286, etc. for various forms of cells.)

There is another form of cell, the _two-fluid_ type, in which _electro-chemical_ means are employed, and in which a metal is deposited upon the - plate instead of hydrogen. The - plate is usually copper, copper being deposited upon it.

=EXPERIMENT 113. To study the "two-fluid" Galvanic cell.=

_Apparatus._ The glass tumbler, G T, (No. 65); porous cup, P C, (No. 73); the strip of zinc (No. 60), well amalgamated (§ 257), or the amalgamated zinc rod (No. 74); piece of sheet copper (No. 75), bent so that it will surround P C; copper wires, C W, with connectors; a saturated solution of copper sulphate, commonly called blue vitriol or blue stone (See § 283); dilute sulphuric acid (See § 258). With the above, set up the two-fluid cell (See § 281). The galvanoscope, G V, complete, is also needed, and if quantitative work is to be done, a pair of scales weighing to 0.1 gram is necessary. (See App. Book,