Part 43 (1/2)

=247. Amalgamation.=--Local action is prevented by coating the zinc with mercury. This process is called _amalgamation_. The mercury covers the entire surface of the plate in the acid. Its action is to dissolve pure zinc and bring it to the outer surface where it is acted upon by the acid. The carbon particles are kept covered so that no local currents can be formed as long as the bits of carbon are below the surface.

Amalgamation therefore prevents local action.

Important Topics

_The Simple Voltaic Cell_

1. Two plates: zinc, copper; electrolyte, dilute sulphuric acid.

2. Ions: hydrogen, positive: sulphion, negative.

3. Current, where and how produced, direction, ill.u.s.tration.

4. Polarization: cure, local action, cure.

Exercises

1. Write in your own words an account of the production of an electric current by the simple voltaic cell. Use sketches.

2. Which plate has the higher potential? How is it produced?

3. Would you expect to get an E.M.F. by forming a cell of two copper plates? Why?

(3) PRACTICAL VOLTAIC CELLS

=248. Advantages of Voltaic Cells.=--Many forms of voltaic cells have been devised. Several of the more common of these will be described and their electro-chemical action explained.

At the present time voltaic cells are employed only where small currents are needed, such as for electric bells and induction coils. Where more than a small amount of current is required, the dynamo and the storage battery have generally taken their place as sources of electric current.

The advantages of voltaic cells as electric generators are: (a) they are inexpensive, (b) they are easily taken from place to place, (c) they may be ready for instant use.

The most desirable voltaic cell would be one having the following qualities: (a) High electromotive force, (b) no polarization or local action, (c) very low internal resistance, (d) small expense, both as to first cost and upkeep.

[Ill.u.s.tration: FIG. 222.--The Leclanche cell, ”wet” type.]

=249. The Leclanche cell= is the one commonly used for ringing door bells. It has two plates: one of zinc and the other of _carbon_. These are placed in a solution of sal ammoniac (Fig. 222). Take up the desirable qualities mentioned at the end of the preceding paragraph. (a) It may be shown that this cell has a good E.M.F. about 1.5 volts. (b) It _polarizes_ easily yet it recovers well when left upon open circuit.

Usually a substance called manganese dioxide is mixed with the carbon.

This acts as a _depolarizer_, that is, it combines with the hydrogen to form water. (c) Its resistance varies and is often considerable. (d) The expense for upkeep is small, since a 5-cent rod of zinc, and a 5-cent charge of sal ammoniac will keep the cell in action on a bell circuit from six months to a year or more. It is well suited for use on _open circuits_ that is, where the circuit is open the greater part of the time and is closed only occasionally; as in ringing door bells, operating telephones, and other devices whose circuits are usually open.

=250. The Dry Cell.=--Many forms of Leclanche cells are made. One of these is called the _dry cell_ (See Fig. 223.) In this cell the zinc plate is made into a jar or can and contains the other materials. At the center of the cell is a rod of carbon and manganese dioxide. The s.p.a.ce between the carbon and zinc is filled with a porous material such as sawdust or plaster of Paris. A strong solution of sal ammoniac fills the porous material. The top of the cell is sealed with pitch or wax to prevent evaporation. The great advantage of this cell is that it may be used or carried in any position without danger of spilling its contents.

Dry cells are often used to operate the spark coils of gas and gasoline engines. The Leclanche cell described in Art. 249 is commonly known as the ”wet cell.”

[Ill.u.s.tration: FIG. 223.--The Leclanche cell, ”dry” type.]

[Ill.u.s.tration: FIG. 224.--The Daniell cell.]