How to Discover Polarity

Radio Record, Volume I, Issue 47, 8 June 1928, Page 12

How to Discover Polarity

HE accurate and speedy determination of the polarity of various aceumulator and high-ten-sion leads is often a matter the paramount importance of which cannot be overemphasised. _ In aceumulator chatging operations the polarity’ of the accumulator and of the charging board leads must, of course, be known with certainty, for it is essential that the positive terminal of the accumulator be connected up to the positive terminal of the charging-board. If the leads were conaccumulator would be completely ex-

nected up the wrong way round, the hausted of its residual charge, and injury would occur to it. SEVERAL CHEMICAL METHODS. GIMILARLY, the radio amateur and experimenter must always be in a position to determine at once the. polarity of his high-tension battery leads. Most high-tension batteries are, of course, at the present moment sent out by their manufacturers with their polarities plainly indicated on the outer casings of the cells, but with home-constructed batteries the case is often different, and the amateur is generally left to himself to determine the polarities of his batteries. Leaving aside the several physical methods of polarity determination, such as the magnetic polarity indicator, there are a number of reliable chemical means of determining very simply the polarity of any electrical lead. A MORE DELICATE INDICATOR. N the first place, a little dilute suiphurie acid may be poured inte a small saucer and the battery lends held in the liquid at a distance of about half an inch apart Bubbles of gas will immediately be observed to rise from the electrode which enrries the negative current. This is, shown at Fig. 1. Similarly, a solution of copper sulphate (blue vitriol) may be prepared, and the wires from the battery dipped below the surfa:e uf this tiquid. In this case, a deposit GE cepper will be formed on the nesasire lead of the battery. . When carrying out this determina-. tion, it is best to wrap the ends. of tlie wires with a little tinfoil so that the. deposit’ of metallic copper -at the negative pole may be shown up plainly. These tests work very well when the E.M.F. of the battery or accumu: | lator cells tested is not below three volts, but there are a number of moré delieate tests still, and many of these, ywhen carefully applied, will indicate the polarity of a cell when its total E.AM.F. is below one volt. Probably the most delicate chemical polarity indicator is the chemical compound which is known by the name of phenolphthalein. This material takes the form ef 2 white powder. A quarter of an ounce of it should be procured from the local chemist and as much of the material as will fit on a sixpence should be dissolved in .a teacupful of warm water, together with about three times as much tlauber’s salts (sodium sulphate. THE USE OF POTATOES. A QUANTITY of ordinary white blotting paper is then soaked

tor a2 few minutes im This quid, and then allowed to dry without any rinsing. The paper may then be cut up into suitably shaped strips. To use this paper for the purpose of polarity determination all that it is necessary to do is to lay a small piece of it on a wooden or ebonite surface. and to moisten it with a little pure water. Bring the leads of the battery on to the moistened surface of the paper and at a distance of about half an inch apart, and a bright pink matk will be eyident at the negative lead. It has been stated that the freshlyeut surface of a potato, an oniou, or | an apple will provide an effective means of determining polarity, a dark stain being produced at the negative pole of the battery when the leads are held on the freshly-cut surface nt a little distance apart. This is a rough-and-ready test, buf ino many cases it works very well. In others, the opposite result is obtained. Thus » it would appear that the chemical ‘or physical make-tip of the vegetable

a or fruit concerned has a good deal to do with the efficiency of the test. Needless to say, this test is not suitable for determining the polarity of yery small currents. The ordinary "pole-finding paper" which is obtainable on the market is usually composed o absorbent paper which has been dyed with a solution of turmeric or litmus. With. both these papers the negative pole of the high-tension battery or accumulator lead exerts a colour change at its point of contact with the moistened paper. These papers can very easily be made by the radio amateur who has a little time to spare. The turmeric variety of the pole-finding paper is perhaps the better of the two to use. It can be made by soaking ordinary white blotting-paper in a solution of turmeric, prepared by dissolving about a quarter of an ounce of dried turmerie root in half a teacupful of a mixture of equal parts of water and methylated spirits. After drying, the paper is ready for use, and when the electrodes. of the battery are applied to the surface of the moistened paper 2 brown stain appears at the negative point of contact. An ordinary photographic blue print will also make quite a good polarity indicator. ¥or this purpose the blue

) Polsrtly Indicators print should be moistened — slightly with a very weak salt solution. The negative point of contact of the battery with the-paper will be indicated by the appearance of a dirty yellow stain. Generally speaking. this test is not very effective when voltages below six or eight are being dealt with. In all these cases, of course, the colour change effect at the point of contact of the negative electrode of the battery is dependent upon the pres--ence of local chemical action by whieh the moist indicating material with which the fibres of the paper are impregnated is converted into. a different form. Sueh colour-change effects ure well-known in the realms of analytical chemistry, and they form the basis of all the well-known chemical "indientors," INSETRUMERY FOR PERMANENT TSE, . N vouslusion, a very sluple polarity indiexstor may be described. This eonsists of a piece of glass tubing as depicted in the diagram, Fig. 2. Through the corks which are provided at the opposite ends of the tube are placed two stout copper wires renching about half an ineh inwards. The tube is then almost entirely filled with a solution of phenophthalein, made as described above. On attaching the polarity indicator to the terminal of the accumulator or battery, a pink area of liquid will be formed around the negative electrode. On shaking the tube, the pink colour will disappear, and thus the indicator will be ready for a further test when required. If the pink colouration is not readily formed, it-is a sign that too little of the materials haye been dissolved in the water. This ean easily be remedied by adding very smali further amounts of.sodium sulphate and phenolphthalein to the liquid until the colour is formed almost at the instant of connecting up the battery. The exact proportions of the materials necessary for the purpose varies to some extent with the voltage of the current whose polarity it is desired to test. Yor ali ayerage purposes, however, the proportions given-above will be found to be ‘entirely ‘suitable, and satisfac tory results will be obtained at the first experiment with the indicator

HANDY form of the above indieator has been devised by | "Megohm" for use with chemical rectifiers, and which allows of a test being made at any.time by merely pressing a key. Referring to the illustration, the key arrangement is seen on the top of a small glass jar of the type that is fitted with a metal. screw-on lid. A square of ebonite and a square of 3-8-inch wood, both 2 by 2 inches are required. Two holes are drilled in the metal lid sufficiently far apart and large enough to easily clear the electrodes passing through. Other holes are made in the lid, and through these are passed small screws to secure it) to the square of wood. OLES are now drilled in the ebonite, including two for screwing down to the wood. ‘The brass strip for the key is springy brass 5 inches long and 4-in. wide, and the brass for the bridge is 1% by 5-16 inches and_ bent to leave a space j-in. in height. The bolt B is connected to the key. by an L-shaped piece of brass fvil or a wire. All but the key may now be bolted to the ebonite, the two 20's copper wire electrodes being bolted to the nut B and lower -contact of key respectively, this contact being a_ bolt lead. With holes drilled through the wood the electrodes may now be passed through and the ebonite secured in-place. the screw holes being clear of the position of the electredes. Depressions are cut in the upper side of the wood to take projecting boits and Wires on the ebonite. The glass jar in question in 2i-in. high nud 1j-in. diameter. IIE indicater ix connected in the positive lead from vectifier to battery, any voltage. at the bolts A and B. and whilst charging. the curvent only traverses fhe key, which presses up against the bridge. When the key is pressed, direct connection is broken. and the-current traverses the liquid. showing a deep red colouration round the negative electrode,which should be the one connected to the positive of battery, This arrangement enables a close witeh to be kept upon a chemical reci tiver with the minimum of trouble. } The liquid, which need not be mere than an inch deep, is the same as that described above.