Motion Picture News (Jul-Aug 1923)

696 Motion Picture News National Anti-Misframe League Forum iMPLiFiED Electricity for Projectionists Storage Batteries j Lesson VI — Part IX Storage Batteries N this modern day of the " radio bug " the storage battery has become a rather well known article, and, from all the writer could observe on his visits to the projection rooms of various theatres, this subject should be more or less Lead peroxip/ate — Container Figure 87 interesting to quite a number of "projectionists— radio hounds!" In spite of their so common use, comparatively few of the users know anything of the underlying principles governing the action of these electrical devices. So live and learn. The storage battery derives its name from the fact that it stores chemical energy which, at some future time, can be changed into the electrical energy. A storage battery, contrary to popular belief, does NOT store electricity. The only electrical device which, in any sense of the word, can be said to do this isa condenser. So remember that it is chemical energy and not electricity which is stored. The electrical energy which results from the chemical action of the storage battery is, as pointed out in a preceding article, in itself useless and must, in turn, be converted into mechanical energy before it can be of any service. Another name, which, perhaps, is not quite so common, is secondary cell. As distinguished from the primary cell described in last week's article, the secondary cell, or storage battery, receives new energy, not from the renewal of its plates and electrolytes (chemical) but rather from the reversal of its chemical action when electricity is applied to its terminals. It is this reversible action of tbe chemicals which makes the storage battery possible. Lead Batteries There are, in the main, two types of storage cells, the lead plate battery and the Edison battery. Both, of course, depend upon chemical action for their operation but the cells themselves are materially different, both in construction and theory. If a plate of pure lead and one of lead oxide (Fig. 87) are placed in a jar containing a solution of sulphuric acid and water, to weaken it, the lead oxide plate will be found to have a higher potential than the lead plate, meaning that the first will be plus and the second minus. The action is as follows: Sulphuric acid -f lead = lead sulphate -f hydrogen gas. The hydrogen carries a negative charge of electricity to the lead oxide plate which it changes to another form of lead oxide (lead monoxide). Hydrogen gas -f lead peroxide = lead monoxide -f water. The sulphuric acid changes this lead monoxide to lead sulphate which remains on the plate. Sulphuric acid -f lead monoxide = lead >ulphate + water. The entire operation can be written in one equation as follows: Sulphuric acid -+ pure lead -f lead peroxide = lead sulphate + water. In other words, this means that, during discharge, acid is taken away from the electrolyte (chemical solution) whereas water is added to it so that the specific gravity (density) of the electrolyte is decreased. Both plates, on the other hand, are changed to lead sulphate. 1 1 the cell is now cliar-jed l>y applying current to it. this action is reversed and sulphuric acid is added to the electrolyte, water is taken from it, thereby increasing the specific gravity. The plates are reformed back to their original condition. The specific gravity forms a sure means of telling when a battery is charged or discharged and is usually expressed as a whole number instead of the fraction which it really is. The weight of a certain quantity of water is taken as one. The /vicAe/ oxide sponyy iron plate Figure 88 weight of an equal volume of sulphuric acid, divided by one, will then be the specific gravity of the acid. In the case of cells for automobile sen-ice it ranges between 1.28 (1280) when the cell is charged to 11.75 (1175) when the cell is discharged. The voltage sometimes gives an indication as to the condition of the cell but this is often misleading. The voltage when the cell 'is fully charged is approximately 2.0 volts and when discharged it drops gradually to 1.7 volts. Any desired voltage can be secured by connecting in series the proper number of cells. The principal disadvantage of lead cells is that extreme care must be taken to see that they are not left discharged for a long time, and must not be overcharged or overdisdiseharged. They also require care in handling and, as shown in Fig. 87, are usually placed in sand filled trays so that the acid dripping from the jars will not cause damage due to burning action. Eflison Battery Another type of cell which is much more rugged and requires less care is that known as the alkali or Edison battery (Fig. 88). If a plate of pure iron and another of nickel oxide are placed in a solution of potassium hydroxide (caustic potash) the result is a battery with the nickel oxide plate 4 and the iron plate — . The action is as follows: When current Hows through the battery, the nickel oxide changes to another oxide, having less oxygen and the iron is oxidized (oxygen is added) to form iron oxide. Iron rust is iron oxide and is formed by the iron when wet, taking oxygen from the water to form the iron oxide which appears as rust. The heavy reddish brown mud, usually found in swamps is still another form of iron oxide. The above action, condensed, is nickel oxide -f iron = another kind of nickel oxide 4 iron ovide. The oxygen from the original nickel oxide is merely transferred to the iron. If current is now applied to the cell the action reverses : nickel oxide -f iron oxide = original nickel oxide -j pure iron. So that the only thing transferred from one plate to the other is oxygen. The nickel oxide, or positive plate, is built op of small steel tubes packed with layers of first nickel hydroxide and flaked metallic nickel. The hydroxide is later changed to nickel oxide. The flaked nickel is added to better conduct than the electricity. The iron or negative plate is composed of a nickel plated steel grid containing small pockets filled with powdered iron oxide. These plates are then assembled in two groups, the positive plates being all connected together and the negative plates likewise. As in the case of the lead battery, there is always one more negative plate than positive. The two groups are then sandwiched together and are placed in a tank of cold rolled steel having welded joints. It cannot be opened for repairs and must be returned to the factory should such be necessary. The Edison battery is lighter than the lead plate type and can be left discharged without damage resulting to the plates. Due to its I 3or 2 0 Z.o c I IS I /■o cho rem tost O J Z 5 4 5 6 7 6 3 /O 7~/me /n Hours Figure 89 construction, it can also withstand very rough handling. Its principal disadvantages are its higher cost and lower voltage, the voltage being about 1.25 volts when fully charged. This of course, necessitates the use of a larger number of cells to obtain a required voltage.