Physics Part 43

=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.]

=251. The Daniell Cell.=--This cell is often used in laboratories, and on closed circuits such as those connected with fire and burglar alarms and telegraph lines. It has two plates of zinc and copper placed in two different liquids which are kept separated by a porous clay cup (Fig.

224). The zinc rod is kept in a solution of zinc sulphate contained in the porous cup. The copper plate is in a solution of copper sulphate filling the rest of the gla.s.s jar. Unlike the Leclanche cell, this one must be kept upon a _closed circuit_ to do its best work, as the two liquids mix when the circuit is open. Taking its qualities in order, (a) its E.M.F. is about one volt, (b) it has no polarization since copper instead of hydrogen is deposited upon the copper plate. Therefore a uniform E.M.F. may be obtained from it, making it especially useful in laboratory experiments and tests. (c) Its resistance is considerable and (d) it is more expensive to operate than the Leclanche. It is sometimes used upon closed circuits outside of laboratories as in burglar and fire alarms, although in recent years, the storage battery is taking its place for these purposes.

=252. The Gravity Cell.=--Fig. 225 is like the Daniell cell in most respects, except that in this cell, the zinc plate is held at the top of the jar in a solution of zinc sulphate while the copper plate is at the bottom, surrounded by a solution of copper sulphate. The solutions mix but slowly as the copper sulphate solution is denser and remains at the bottom. This cell like the Daniell must also be kept upon closed circuit. On account of its simplicity and economy it is often used to operate telegraph instruments. Its qualities are similar to those of the Daniell cell.

[Ill.u.s.tration: FIG. 225.--The gravity cell.]

=253. Symbol for Voltaic Cells.=--In electrical diagrams, the symbol employed to represent a voltaic cell is a short thick line near to and parallel to a longer thin one. As in Fig. 226. If several cells are to be represented the conventional symbol of the combination is represented as in Fig. 227. A single cell and a group of cells are each frequently called a battery.

[Ill.u.s.tration: FIG. 226.--Diagram of a single cell.]

[Ill.u.s.tration: FIG. 227.--Diagram of a group of cells.]

=254. Effects of Electric Currents.=--Having studied some of the devices for producing an electric current, let us now consider some of the _effects_ caused by it. These effects will be studied under three heads: (a) _Magnetic_, (b) _Chemical_, and (c) _Heat_ effects. Devices or articles showing these effects known to most high school students are respectively: (a) the _electromagnet_ (b) _electro-plated silver ware_ and (c) _electric heaters_, such as electric flat irons, electric toasters, etc. The _magnetic_ effect of an electric current was first detected by Oersted at the University of Copenhagen in 1819. It may be observed by holding a wire carrying a current from a voltaic cell above and _parallel_ to the needle of a _magnetoscope_. The needle is at once deflected (Fig. 228). If the current is reversed in direction the magnetoscope needle is deflected in the reverse direction. This simple device is the most common means for detecting an electric current. It therefore const.i.tutes a _galvanoscope_. (See Art. 239.)

[Ill.u.s.tration: FIG. 228.--A galvanoscope.]

Important Topics

1. Leclanche cells, (a), wet, (b), dry, construction, advantages, uses.

2. Daniell and gravity cells, construction, advantages, uses.

3. Three effects of electric currents, ill.u.s.trations.

4. The galvanoscope, uses.

Exercises

1. Explain how the direction of current in a wire can be determined by a compa.s.s.

2. Would you expect to obtain a current from a zinc and copper cell containing a solution of common salt? Perform the experiment.

3. What conditions in a voltaic cell will give a steady electromotive force.

4. What conditions in a voltaic cell will give a strong electromotive force.

5. Name three different electric circuits that you know exist. Which are _open_ and which are _closed_ circuits?

6. Are voltaic cells used in your home? If so, for what purpose are they used? On open or closed circuits? Have you seen them? what kind are they?

CHAPTER XII

THE MAGNETIC EFFECT OF ELECTRIC CURRENTS. ELECTRICAL MEASUREMENTS