Primary Batteries

Radio Record, Volume II, Issue 25, 4 January 1929, Page 27

Primary Batteries

Make Useful Standbys

@F recent years the primary cell in many of its various forms has died a natural death. The storage battery or accumulator has rightly taken its place, but even now there are times when a primary. battery could be. brought to service in the absence of an accumulator, There is one form of primary cell ‘hat has not gone out of fashion and is as popular to-day as ever, That is the Leclanche, The old jar type of cell is still used in its original form for power to operate electric bells, signalling devices, etc. The well-known dry battery is really a dry (or nearly dry) Leclanche, which has the electrolyte made into a jelly and enclosed in a zinc container. A few weeks ago the writer had . misfortune to have a run-down attery on the very evening when visitors were expected. Of course it is an old tale to have a run-down battery when the set is not going too well, but the complaint was genuine this time and there were no facilities for getting a charge that would suffice even for a few hours. Electricity is just the same whether derived from an accumulator or from a primary battery, so a few inutes were spent in hunting ingredients likely to be useful. A bichromate battery was aimed at first, but this was cast aside, as there was no bichromate of potash. Instead a permanganate of potash battery was made which, though unorthodox, saved the situation and, in fact, has been in use since. A Good Primary Battery. ERD are given the details for making this battery, which can be asiembled in a few minutes, maybe helpimg other constructors out of similar difficulties. Each cell gives an E.M.F. of 1.8 volts, which drops to 1.65 after a few hours’ continuous run, so the total voltage will have to be made by joining two or more cells in series. To make one cell, procure a porous pot, a zine rod and carbon rod. An old wet Leclanche pot can be cleaned out and will provide the porous pot and carbon rod. Next get a glass or earthenware jar of sufficient size to allow the porous pot and the carbon rod to stand side by side. Now fill the porous pot and jar about three-quarters full of sulphurie acid, strength 10 per cent. This is made by pouring one part of sulphurie acid by volume into parts of water. Pour in slowly, stirring meanwhile, and taking care that the heat generated will not crack the container. ways pour the acid into the water en diluting sulphurie acid. The level of acid is the same inside the porous pot as in the outer jar. Crystals of permanganate of potasb are now dropped into the jar in the acid between the porous pot and jar, in which is standing the carbon jar. Operating the Battery. S$ the solubilitv of this compound is 1 in 20, about an ounce of permanganate will be sufficient. The cell is now ready to operate and all that remains is to insert the zine rod inside

the acid in the porous pot all the time the cell is in operation. When not in use the zine rod must be lifted out and laid aside until the cell is required again. To prolong the life of the zinc, a small quantity of metallic mercury can be run into the bottom of the porous pot. A spot or two is sufficient and will give the zine a coating of mercury. By employing a saturated solution of potassium bichromate in the 10 per cent acid in the outer jar, the true bichromate cell which has certain advantages, is made. The chief advantage of doing this is that the cell is odourless in operation, while the one without the solution of potassium bichromate gives off a slight chemical smell. It is as well to remember that the zine rod is negative and carbon positive in all primary cells. To increase the output amperage of this cell the internal resistance has to be lowered. This is obtained by using a larger carbon surface. Two carbon rods connected together give nearly double the current output, but maintain the same voltage. Theory of Operation. HB theory of operation of all primary cells is practically the same in‘each case. When a zinc and carbon rod are immersed in sulphuric acid and the two projecting ends are connected together with a conductor, hydrogen "ions" are liberated from the zinc and pass over through the liquid to the carbon rod. These "ions" are considered to be very small charges of electricity, carrying very minute particles of the gas hydrogen with them. These particles of hydrogen are too small to form a bubble and rise to the surface, so they collect on the carbon rod and very soon form a layer of insulating gas. This causes a reduction in the current and the cell is known as "polarised." This polarising action of a simple cell, of zinc and carbon in sulphuric acid, takes only a few minutes, and in this form it is useless except for the supply of current of a very intermittent nature. Some depolarising agent has to be used, and the different chemicals used for taking out the hydrogen constitute the different types of primary cell. The principal type is the use of an oxidising agent. This contains active oxygen, which combines with the hydrogen to form water, a harmless compound. Permanganate of potash, bichromate of potash, and concentrated nitric acid areal] oxidising agents, and are all used in the container outside the porous pot. The Leclanche employs manganese dioxide, which also liberates free oxygen. A further action also takes place in the Leclanche. Manganese dioxide is known as a "catalytic" agent, which denotes a substance which helps two other compounds to unite. In this case- the other two are nitrogen and hydrogen, forming ammonia. Salammoniae or ammonium chloride is used in Leeclanche cells, and when in contact with the zine rod, hydrogen and nitrogen are liberated, which combine

in the porous pot to form merely ammonia. The ions have then been robbed of their hydrogen contents and fly to the carbon rod above forming, in their millions, the current of electricity. No layer of insulating hydrogen bubbles form on the carbon, and therefore a continuous stream of current is obtainable until either the zinc or acid wear out.