If the n cells are connected in parallel, that is. coppers are connected together and all the zincs, asi the E.M.F. of the combination is only the E.M.F. cell, while the internal resistance is 1/n of that cell, since connecting the cells in this way is simply to multiplying the area of the plates n times. 1 furnished by such a

copper wire, the current flowing through it will be great as that which could be made to flow throug single cell. Figs. 288 and 290 show by means of analogy why the E.M.F. of cells in series is the s several E.M.F.'s, and why the E.M.F. of cells in par greater than that of a single cell. These considerat that the rules which should govern the combination as follows: Connect in series when R is large in comp R; connect in parallel when R; is large in comparis

ut o

ce

i

ESTIONS AND PROBLEMS

having an E. M. F. of 1.5 volts .. rough an ammeter having a negli resistance of the cell.

ché cell better than a Daniell cell ·

wires in series and three cells in se ce of .1 ohm, what is the resistance of esistance of .1 ohm, what is the internal

K

K2

B

МЕНИНИЦШ

low-resistance galvanometer being inserted in the circuit. As the current flows, hydrogen bubbles will be seen to rise from the cathode (the plate at which the current leaves the solution), while the positive plate, or anode, will begin to turn dark brown. At the same time the reading of the ammeter will be found to decrease rapidly. The brown coating is a compound of lead and oxygen, called lead peroxide (PbO2), which is formed by the action upon the plate of the oxygen which is liberated, precisely as in the experiment on the electrolysis of water (§ 302). Now let the batteries be removed from the circuit by opening the key K1, and let an electric bell B be inserted in their place by closing the key K2. The bell will ring and the ammeter A will indicate a current flowing in a direction opposite to that of the original current. This current will decrease rapidly as the energy which was stored in the cell by the original current is expended in ringing the bell.

FIG. 291. The principle of the storage battery

This experiment illustrates the principle of the storage battery. Properly speaking, there has been no storage of electricity, but only a storage of chemical energy.

Two similar lead plates have been changed by the action of the current into two dissimilar plates, one of lead and one of lead peroxide; in other words, an ordinary galvanic cell has been formed, for any two dissimilar metals in an electrolyte constitute a primary galvanic cell. In this case the lead peroxide plate corresponds to the copper of an ordinary cell, and the lead plate to the zinc. This cell tends to create a current opposite in direction to that of the charging current; that is, its E. M. F. pushes back against the E. M. F. of the charging cells. It was for this reason that the ammeter reading fell. When the charging current is removed, this cell acts exactly like a primary galvanic cell and furnishes a current until the thin coating of peroxide is used up. The only important difference between a commercial storage cell (Fig. 292) and the one which we have here used is that the former is provided in

the making with a much thicker coat of the "active material " (lead peroxide on the positive plate and a porous, spongy lead on the negative) than can be formed by a single charging such as we used. This material is pressed into interstices in the plates, as shown in Fig. 292. The E. M. F. of the lead storage cell is about 2 volts. Since the plates are always very close together and may be given any desired size, the internal resistance is usually small, so that the currents furnished may be very large.

The usual efficiency of the lead storage cell is about 75%; that is, only about as much electrical energy can be obtained from it as is put into it.

339. Nickel-iron storage batteries. Thomas A. Edison (see opposite p. 316) developed and perfected the nickel-iron caustic-potash storage cell. The electrolyte is a 21% solution of caustic potash in water. The negative plates contain iron powder securely retained in perforated flat rectangular capsules, while the positive plates contain oxide of nickel in perforated cylindrical containers. For equal capacities the Edison cell weighs about half as much as the lead cell, and it will stand a remarkable amount of electrical and mechanical abuse. The E.M.F. is about 1.2 volts. In efficiency it is a little below the lead cell. Caustic potash is now replaced by caustic soda.

QUESTIONS AND PROBLEMS

1. In charging a storage battery is it better to say that the current passes into the cell or through it? What is "stored "?

2. The lead peroxide plate and the nickel oxide plate are both called "the positives." What is the relation of the charging current to these plates?

3

HEATING EFFECTS OF THE ELECTRIC CURRENT

340. Heat developed in a wire by an electric current. Let the terminals of two or three dry cells in series be touched to a piece of No. 40 iron or German-silver wire and the length of wire between these terminals shortened to 4 inch or less. The wire will be heated to incandescence and probably melted.

The experiment shows that in the passage of the current through the wire the energy of the electric current is transformed into heat energy. The electrical energy expended when a current flows between points of given P.D. may be spent in a variety of ways. For example, it may be spent in producing chemical separation, as in the charging of a storage cell; it may be spent in doing mechanical work, as is the case when the current flows through an electric motor; or it may be spent wholly in heating the wire, as was the case in the experiment. It will always be expended in this last way when no chemical or mechanical changes are produced by it. (See drawings opposite p. 269 for uses made of heating effects.)

We found

341. Energy relations of the electric current. in Chapter IX that energy expended on a water turbine is equal to the quantity of water passing through it times the difference in level through which the water falls; or, that the power (rate of doing work) is the product of the fall in level and the current strength. In just the same way it is found that when a current of electricity passes through a conductor, the power, or rate of doing work, is equal to the fall in potential between the ends of the conductor times the strength of the electric current. If the P.D. is expressed in volts and the current in amperes, the power is given in watts, and we have

volts × amperes

=

watts.

The energy of the electric current is usually measured in kilowatt hours.