Motion Picture News (Apr - Jun 1928)

2130 Motion Picture News Projection Optics, Electricity, Practical Ideas & Advice Inquiries and Comments Storage Batteries and Their Care IPROJECTIONIST in Southern Illinois recently wrote to us stating that he was responsible for the care and operation of a bank of storage batteries placed in his theatre in connection with an emergency lighting outfit. Not being overly familiar with such devices he requested information as to their general construction and operation in order that he be able to give them intelligent attention and assure their satisfactory performance. This we promised to do. The storage battery derives its name from the fact that it stores chemical energy which, at some future time, ean be changed into electrical energy. A storage battery, contrary to popular belief, does not store electricity. The only device which, in any sense of the word, can be said to do this is a condenser. So remember, that it is chemical energy and not electricity that is stored. The electrical energy which results from the chemical action of the storage battery is, in itself, useless and must be, in turn, converted into mechanical energy before it can be of any use. Another name for the storage battery is secondary cell. As distinguished from the primary cell, which actually generates electricity, the storage battery receives new energy, not from the renewal of its plates and electrolyte (chemical) but rather from the reversal of the chemical action when electricity is applied to its terminals. It is this reversible action of the chemical which makes the storage battery possible. Types of Batteries There are, in general, two types of storage batteries : the lead plate cell and the Edison cell. Both, of course, depend upon chemical action for their operation but the cells themselves are materially different, both as to construction and method of operation. If a plate of pure lead and one of lead oxide 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 lea<J = lead sulphate -| 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 -| lead peroxide = lead monoxide -f water. The sulphuric acid changes this lead monoxide to lead sulphate which remains on the plate. Sulphuric acid + lead monoxide = lead sulphate + water. The entire operation can be expressed in one equation as follows : Sulphuric acid -\ pure lead -| lead peroxide = lead sulphate -| water. In other words, this means that, during dicharge, acid is taken away from the electrolyte (solution) whereas water is added to it so the specific gravity (density of the electrolyte is decreased. Both plates, on the other hand are changed to lead sulphate. If the cell is now charged by 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 returned to original condition. The specific gravity is 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 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 service 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 may be obtained 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 overdischarged. They require care in handling and are usually placed in sand filled trays so that the acid dripping from the jars will not cause damage due to burning action. The Edison Battery Another type of cell which is more rugged and requires less care is that known as the alkali or Edison Battery. 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 -\ and the iron plate — . The action is as follows : when current flows through the battery, the nickel oxide changes to another oxide, having less oxy gen and the iron is oxidized (oxygen is added) to form iron oxide. Iron rust is iron oxide and is formed when the iron is 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 -| iron — another kind of nickel oxide + iron oxide. 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 -| pure iron. The nickel oxide, or positive plate, is built up of small steel tubes packed with layers of first, nickel hydi-oxide and flaked metallic nickel. The hydroxide is later changed to nickel oxide. The flaked nickel is added to better conduct the electricity. The iron, or negative plate is composed of a nickel plated steel grid containing small pockets filled with powdered iron oxide. The 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 construction, it can also withstand very rough handling. Its principal disadvantages are its higher cost and lower voltage. The voltage per cell is about 1.25 volts when fully charged. This, of course, necessitates the use of a larger number of cells to obtain a required voltage for operation. New Theatre Under Way At Marion, N. G. E. J. House, owner and manager of the Oasis Theatre, Marion, N. C, who, a short time ago was reported as planning to erect a theatre on the outskirts of the city, in what is known as the mill section, now announces that he has organized a stock company, in which he will hold the control ling interest, which will build a thoroughly modern theatre in the heart of Marion. The plans which are now being prepared, Mr. House states, call for a seating capacity of 720, with sufficient height to the building to permit the installation of a comfortably sized gallery, should one be needed later.