Part 43 (2/2)

=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

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