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Test # 9


1.
The ratio of third proportional to 12 and 30 and the mean proportional of 9 and 25 is :

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Quantitative Aptitude
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  • Simple Science

    Experiments which grew out of the Voltaic Cell

    Electricity:
    Since chemical action on the zinc is the source of the current, it would seem reasonable to expect a current if the cell consisted of two zinc plates instead of one zinc plate and one copper plate. But when the copper strip is replaced by a zinc strip so that the cell consists of two similar plates, no current flows between them. In this case, chemical action is expended in heat rather than in the production of electricity and the liquid becomes hot. But if carbon and zinc are used, a current is again produced, the zinc dissolving away as before, and bubbles collecting on the carbon plate. By experiment it has been found that many different metals may be employed in the construction of an electric cell; for example, current may be obtained from a cell made with a zinc plate and a platinum plate, or from a cell made with a lead plate and a copper plate. Then, too, some other chemical, such as bichromate of potassium, or ammonium chloride, may be used instead of dilute sulphuric acid.

    Almost any two different substances will, under proper conditions, give a current, but the strength of the current is in some cases so weak as to be worthless for practical use, such as telephoning, or ringing a door bell. What is wanted is a strong, steady current, and our choice of material is limited to the substances which will give this result. Zinc and lead can be used, but the current resulting is weak and feeble, and for general use zinc and carbon are the most satisfactory.

    FIG. - A simple electric cell.


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