Simple Polarity Indicators

Radio Record, Volume III, Issue 29, 31 January 1930, Page 37

Simple Polarity Indicators

Some Easily-applied Tests —

Wits the present-day fairly general use of battery chargers and mains units a simple method of determining the correct polarity of mains becomes &@ necessity. An accumulator Incorrectly connected to a charger and left in this way for a few hours may be permanently ruined. In the case of B battery eliminators operating . from A.C, mains the output terminals are, in’ most cases, marked in some way. With ‘certain commercial chargers the practice is to bring out flexible leads and stamp the connecting spade tags. Sooner or later these tags become corroded, and it is difficult to determine the positive and negative leads. Some form of polarity indicator would therefore appear to be a necessity for every power-from-the-mains wireless user. It is extremely difficult to determine from inspection of twisted flex. the relative polarity of the two leads. The easily-applied tests include (1) voltmeter; (2) water test; (3) potato test; (4) chemical test. In the first type a voltmeter of the moving-coil type may be used, for with this class of meter a reading is only obtained when the positive of the supply is joined to the positive. terminal _on the meter. If the meter is incortectly connected the needle will tend to move in the wrong direction. The water test is quite simple, but it is apt to be misleading. The output leads of unknown polarity are placed in a slightly acidulated solution about one inch apart. Bubbles of gas will be given. off from the ends of both wires, but .at one end the gas is produced, in much greater quantities. This lead is joined to the negative terminal of the supply. -In practice the faults to. be found with this test are: it is messy, and it is sometimes difficult to determine the lead from which the gas is liberated in the greatest quantity. Should it be necessary to test polarity across two points between which a high voltage exists, a resistance, such as a lamp, must be inserted in series with one of the leads to prevent a possible short. A freshly-cut potato with the two led stuck a small distance apart is the "third type of polarity indicator. The current passing through the potato between. the two ends. of the wire causes a green stain to he left on the wire connected to the positive of the supply. But as a certain amount of ‘mark is also often left on the negative lead, this test does not give as definite an indication of the polarity as could be desired. The last and most effective polarity indicator is the purely "chemical" type. Indicators of this type are extremely, easy to construct, they are ‘practically indestructible, and give a definite indication°as to polarity. Obtain from a chemist -some sodium sulphate and some phenolphthalein (about three-penny-worth of each). Half-fill an old tumbler or cup with warm water, and into this place as much of the former substance as will cover two pennies and sufficient phenolphihalein

‘to cover a sixpence (not six pennies), and stir. It will be found that these two chemicals will not dissolve, but merely form a suspension. Test the solution by placing two leads from a 2, 4, or 6-volt accumulator into it (about lin. apart), and it will be seen that the negative lead turns the surrounding liquid a reddish colour. A slight shake causes the colour -to disappear. By carefully adding very small quantities of each chemical in turn a point will be arrived at where the liquid is sensitive to 2 volts, . A suitable container for the liquid the next consideration, and for this a length of glass tubing 2in. long with a fin. bore, two small corks, and two terminals are required, ‘To increase the surface contact, small pieces of sheet nickel, brass, or copper are’ soldered to the,ends of the terminals. After filling the tube with. the solution, leaving a space so that the liquid can be shaken to disperse the colour, _earefully seal the corks with paraffin wax or Chatterton’s compound. The excess liquid should not. be thrown away, for with this pole-finding paper can be made. Obtain some good quality white blotting paper and cut into thin strips, immerse in the liquid until thoroughly impregnated, and then hang up to dry. Do not attempt to dry in front of a fire. To use, moisten the paper slightly and then place the wires about din, apart on the wet portion, when the negative lead will leave a red mark.