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Coulometer

silver, water, current, bowl, tube, metallic, plate, set and gas

COULOMETER, in electricity, an instru ment for determining the quantity of electric current which is passing through an electric cir cuit. In other words it is a measure of current efficiency. Several forms of coulometer have been proposed and many tests made with them. Three distinct principles have been employed; the iodine and the iron coulometers depend upon the chemical changes taking place as the result of the current; the copper and the silver cou lometers depend upon the weight of a metallic deposit made by the current ; the oxy-hydrogen gas coulometer depends upon the dissociation of water by the current. The last named was the earliest and is the simplest to operate. The figure shows this gas coulometer to the left con nected with a battery which is being tested. A circular vessel has two wires let through its bot tom, which bear inside two electrodes of plat inum foil as shown ; water slightly acidulated with sulphuric acid is poured into the vessel, and two tubes which have been filled with water are placed over each electrode. On the poles of the battery being connected with the exterior ends of the wires by means of binding-screws, water will be decomposed and hydrogen will appear in one tube and oxygen in the other. It will be found, as represented in the figure, that more than twice the quantity of water is dis placed in the hydrogen tube than in the oxygen tube. Two volumes exactly of hydrogen and one volume of oxygen are the proper proportions to form water; but some of the liberal oxygen in the form of ozone is dissolved by the water. One of the drawbacks of this type of instru ment is that it requires a certain minimum strength of current before it begins to operate.

An improved form of the oxy-hydrogen coulom eter is the Walter-Neuman single-tube in strument. Instead of separating the two gases, it allows both to flow into the one tube, which is placed upright and graduated. The upper part, which is quite long, is about five-eighths of an inch in diameter and has a stop-cock valve at the top. This is used when the gases are taken to an eudiometer for more •accurate measurement. The lower end of the tube is ex panded into an oblong bulb of considerable content as compared with the smaller tube. The electrodes have each an area of one square inch and are set in the bottom of the bulb. A leveling tube is attached• to ensure accurate reading. The advantages claimed for the gas coulometer are that it may be read instantly at the close of the test and that it has a smaller margin of error in that the amount of gas by which the measurements are made is so much larger pro portionally than the metallic deposits or volu metric changes which must be relied on in the other instruments.

The so-called coulometer,"' however, has the largest acceptance among scientific men, although the records show an almost endless 'series of disagreements as to extreme accuracy in the results obtained by its use by different operators. It was formally adopted by the

International Electrical Congress, held in Chi cago in 1203, as the most dependable of all the coulometers, and that congress fixed upon the deposition of silver as the official measure of the efficiency of an electric current, making the international ampere that unvarying volume of current which, when passed through a standard aqueous solution of silver nitrate, shall deposit metallic silver at the rate of 0.001118 of a gramme per second?' In the approved form of the silver coulome ter the cathode is a small platinum bowl, and the anode is a circular silver plate. The latter is smaller in diameter than the bowl, and is riv eted to a silver rod by which it is raised out of the bowl or lowered into it. The platinum bowl is set upon a clean copper plate which is connected with the appropriate pole of the bat tery to be tested. The other pole is electrically connected with the silver rod. Before begin ning the test the bowl is washed scrupulously clean of metallic silver with nitric acid, and then with water and with absolute alcohoL It is then dried at a temperature of 320° F. and accurately weighed. The bowl is then nearly filled with a neutral solution of nitrate of silver in distilled water 15 parts nitrate to 85 parts water. The silver plate is wrapped with a piece of filter paper in such a way that no sediment falling from it during the operation can fall into the bowl. The current is then turned on and the exact time noted. It is allowed to run not less than half an hour. When it is turned off the time is again noted. The silver plate is raised out of the liquid and the latter poured out. The deposit in the bowl is then washed with distilled water and left to soak in it for six hours at least. It is then rinsed with dis tilled water, and then with absolute alcohol and dried at 320°, after which it is allowed to cool in the drier, and then again weighed. The dif. ferenee between the two weights will show the amount of silver deposited. This weight, in grammes, divided by the number of seconds during which the operation is continued, and then by the constant 0.001118, gives the number of coulombs of current ill the circuit tested. A modification of this apparatus makes use of a porous cup set within the platinum bowl and containing a bed of granulated metallic silver upon which the silver plate is set down firmly. The porous cup effectively prevents the passing of any of the "slimes which accumulates among the silver granules, and which, being metallic silver, would tend to vitiate the test. Consult United States Bureau of Standards, 'Bulletin 3' (Washington 1905). and 'Bulletin 8' (1911).