Page:Popular Science Monthly Volume 92.djvu/168

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��Popular Science Monthly

��tive battery groups are shown in Fig. 3, 4 and 5. For all ordinary work, the method of connections in Fig. 3 is em- ployed, where the voltage of the battery is four times as great as that of a single cell. If the same four cells are grouped as in Fig. 4, the voltage will be but twice that of one cell, but the strength of the current will be twice that of Fig. 4. When arranged in parallel, as in Fig. 5, the E. M. F. will be equal to that of one cell, and the current four times that obtained when the cells are connected in series. The voltage of a number of cells in series is equal to the voltage of one cell multiplied by the number of cells.

The voltage obtainable from any cell is independent of its size or of the dis- tance between the plates. For any given cell, however, the current is directly proportional to the size of plates, and inversely to the distance between them. Thus, the distance between the plates affects the current only, as it increases the internal resistance.

The resistance of a number of cells in series is equal to one cell's resistance multiplied by the number of cells. When arranged in parallel the total resistance is equal to the total resistance of one of the cells divided by the number of cells in parallel.

Primary cells are divided into two classes. One class is suitable for con- tinuous work only, and will quickly run down unless connected in the circuit; this is the closed-circuit type. The other will rapidly deteriorate when continually used; this is the open-circuit type.

The best known of the closed-circuit type is the gravity cell, shown in Fig. 2. The positive pole, or cathode, consists of copper located at the bottom of the jar, and the negative pole, or anode, of zinc crowfoot arranged at the top. Both are immersed in a copper-sulphate solution. This type is suitable only for such work as telegraphy, or wherever small currents are used, since the internal resistance of the cell is great.

Open-circuit cells are much more ex- tensively used, including nearly all the different makes of dry batteries. The so- called dry battery consists of an outer cylindrical cup forming the zinc electrode, which is lined with thick absorbent paper and packed with a pulverized manganese

��dioxide and carbon mixture surrounding the central carbon rod. The whole is saturated with ammonium chloride solu- tion and sealed with pitch to keep it from drying out.

Apart from dry cells, the Leclanche cell is most used. In this cell the cathode is of carbon immersed in sal ammoniac solution, and the anode is a bar of zinc immersed in the same liquid, but insu- lated from the carbon. Such cells can deliver a strong current for a short time. If left in circuit, however, they will run down in a short time. These cells are universally used for bell and telephone work, and in places where intermittent current is desired, as they consume no energy when not in use.

(To be continued)

��A Simple Method for Determining Condenser Capacity

A QUICK and easy method of calcu- lating condenser capacity by simple arithmetic, will appeal to all experi- menters, and particularly to those of the younger class who have not reached that stage in the study of mathematics at which they are able to handle formulae. The curves shown here may be used. As an example of their use, suppose it

��CAPACITY PER SQUARE INCH '/4 AIR DiELtCTRIC

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��The curve shows that a condenser will need almost 10 square inches of active dielectric

is desired to build a mica stopping con- denser of .002 mfd. capacity, and the mica available for use as the dielectric measures 8 mils in thickness. From the

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