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Page:Popular Science Monthly Volume 89.djvu/807

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Popular Science Monthly

��793

��A New Method of Reviving Old Dry Batteries

N( >\V that all sorts of electric pocket llashiights and electric lanterns are common, it is worth while to know that the batteries for them need not be thrown away immediately after the light loses its original brilliancy. When

���Three partly exhausted cells are connected, two in series and the third across terminals

a dry cell is so far exhausted that it produces nothing more than a quickK- fading glimmer of light, which it would t)rdinarily keep at full brilliance, it is usually discarded as worthless. Such batteries may, however, be set aside until a number of them have accumu- lated; and some of them may be used to revive the others by the following method, which is applicable to the dry battery because it possesses, to a certain degree, the jiropcriy of reversibility.

When three partly exhausted cells have been accumulated, connect two of them in series, and the third one across their terminals in reverse series, as shown in the drawing. When this is done, it will be seen that the electro- motive force of cells A will more than balance that of cell B; and a small current will tlow through the entire battery, passing through cell B in the reverse direction from normal. The reversed cell, therefore, becomes vir- tually a storage cell, and slowly takes on a charge from the other batteries. After being left in this position for a few da\s, cell B may be disconnected and again used in the regular wa>'. .\

��cell thus reviveil will be as good as new.

M<ire than three cells may be con- nected ; for example, three cells A , may be set to reviving two cells B; in which case the action will be slower because the electromotive force of the odd cell, A, must overcome the internal resistance of five cells instead of three, as in the first case.

Old dry cells for any other purpose may be revived in the same way. They may be tested for exhaustion by con- necting them to an ammeter or to some piece of apparatus such as a bell or telegraph-sounder, and comparing the effect produced by one of the old cells with that produced by a fresh cell when connected to the same. As an ammeter |:)Uts a cell on practically a short circuit, it should be used quickly to avoid further exhaustion of the strength of the cell. The property of reversibility is not possessed by all types of primary Ijatteries; and it must be borne in mind that this article has reference only to the dry cell. — Edwix C. Wright.

��A Home-Made Fuse for a Small Battery Current

THE beginner in electrical science who wishes to demonstrate the action of a fuse on a small scale will find the following a suitable form to use. Make a little wood block .1, 2 in. long by 1 3-2 in- wide, and fix two brass springs, B, one at either end. The screws used to secure these also serve for the attaghment for connecting wires.' Take a i-in. length of glass tube, C, and cork both ends, pasting a very

���Home-made fuse for battery circuits

��narrow strip of tinfoil D — y^ in. or less — along the side and covering both corks with a wider piece of foil. The tube can then be gripped between the springs. This forms a good fuse for small currents.

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