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Fig. 15d.—Method of Charging Storage Battery. storage battery is shown at Fig. 15d, in which a bank of lamps regulates the current strength passing through it from a direct current line. In the larger installations, it is important to carefully regulate the charging rate by means of a suitable rheostat that can be used instead of the lamp bank. Care must be taken to provide a current of the proper voltage, and the charging current passing through the battery should always have higher voltage than that of the battery being charged.

The capacity of storage batteries is regulated by a number of variables but is usually expressed in ampere-hours, though the strength of the

charge is measured in volts. I fully charged battery should show about 2.2 volts per cell. Dry cells are measured with an amperemeter and average cells show 15 to 20 amperes on test. When dry cells indicate less than 5 amperes their action is apt to be irregular and they should be replaced with new ones.

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The Fuller Cell, Fig. 16, is especially well adapted for telephone work. It consists of a glass jar a containing a porous cup c in which is the zinc 2. The porous cup is suspended from the top of the jar and is filled with diluted sulphuric

acid or a salt solution. The salt solution is made by mixing about 3 ounces of salt in a pint of water. The negative plate is carbon in the form of a cylinder e that surrounds the porous cup, and this also is suspended from a wooden or rubber cap s which closes the jar. The jar is filled with a solution of 3 parts bichromate of potash, 1. part sulphuric acid, and 9 parts water. About 2 ounces of mercury is placed in the porous cup with the zinc, for amalgamation; that is, the mercury combines with the zinc and eats the impurities from its surface, causing the zinc to present a clean surface to the acid. The binding screw m, attached to e, and the binding screw n fastened to z, form the terminals of the cell. The internal resistance is about 0.5 ohm, and the electromotive force is 2 volts.

The Gravity Cell, Fig. 17, has for its positive plate three sheets of copper fastened together as shown at c; these are spread out, and set on edge in the bottom of the glass jar m. A gutta-percha insulated copper wire a soldered to the copper extends up through the cell, forming one of its terminals. The negative zinc plate is cast with projecting fingers as shown at 2, and on account of its general resemblance to a crow's foot this cell is sometimes called a “crow's-foot” battery. The zinc is suspended across the top of the jar and is provided with a binding screw as indicated. The solution used is copper sulphate, sometimes called blue vitriol, and water. It is formed by placing in the bottom of the jar from 6 to 8 ounces of coppersulphate crystals n, and then introducing water until the zinc is well covered. A saturated solution of copper sulphate forms around the copper and, after use, a zinc-sulphate solution forms around the zinc and floats upon the copper-sulphate solution. The two solutions, having different specific gravities, are thus kept apart, and as the copper

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Fig. 17.— The Gravity Cell sulphate solution has a deep blue color while the zinc sulphate is light colored, it is easy to distinguish them. The dividing line between them is called the blue line, and the name “gravity cell” results from the part gravity plays in the performance of the liquids. The internal resistance can be reduced, and the cell made immediately available for use after setting up, by using, instead of water, about half a pint of zinc-sulphate solution

from a battery already in commission or by pouring into the water 4 or 5 ounces of pulverized sulphate of zinc. The internal resistance of the cell is from 2 to 3 ohms, and its electromotive force is practically constant at 1.08 volts. Owing to the high internal resistance it is seldom used in

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Fig. 18.—The Edison or Gladstone Cell

telephony except for the transmitter at the switchboard.

The Edison Cell, Fig. 18, employs two zinc positive plates c and e, and a slab a of pressed copper oxide for the negative plate. The plates are suspended side by side from the cover of the jar, the copper-oxide plate being held between the two zinc plates as shown. The jar is filled with a solution of caustic potash and water, on which is

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