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Electromotive Force, commonly denoted by the letters E.M.F. corresporids, to employ a simple analogy, to the pressure or quantity tending to produce motion, as in the case of water, for example, we speak of water in a pipe as exerting a pressure of so many pounds to the square inch ; in a similar manner the E.M.F. of an electric current is that pressure or force tending to produce motion, which causes the current to traverse certain paths.
The practical unit or standard of electromotive force is the volt. There is no battery or constant source of current which yields exactly one volt of E.M.F. between its terminals, the usual standard employed in this connection is therefore Clark's Standard Cell, the E.M.F. of which is approximately 1.454 B.A. volts at 15 degs. C.
The Clark's cell possesses electrodes of pure zinc, and similarly pure mercury, exposed to the voltaic action of a saturated solution of mercurous sulphate and zinc sulphate in water. As a matter of fact, the above-named salts are made up in the form of a paste, beneath which is placed the mercury, whilst the zinc element is embedded in it.
The Board of Trade rules for preparing the cell are as follows:
Definition of the Cell. The cell consists of zinc, or an amalgam of zinc with mercury, and of mercury in a neutral saturated solution of zinc sulphate and mercurous sulphate in water, prepared with mercurous sulphate in excess.
“Preparation of the Materials. “1. The Mercury.—To secure purity it should first be treated with acid in the usual manner, and subsequently distilled in vacuo.
“2. The Zinc.—Take a portion of a rod of pure redistilled zinc, solder to one end a piece of copper wire, clean the whole with glass-paper or a steel burnisher, carefully removing any loose pieces of the zinc. Just before making up the cell dip the zinc into dilute sulphuric acid, wash with distilled water, and dry with a clean cloth or filter paper.
* 3. The Mercurous Sulphate.—Take mercurous sulphate, purchased as pure, mix it with a small quantity of pure mercury, and wash the whole thoroughly with cold distilled water by agitation in a bottle; drain off the water, and repeat the process at least twice. After the last washing, drain off as much of the water as possible.
“4. The Zinc Sulphate Solution.—Prepare a neutral saturated solution of pure ('re-crystallised') zinc sulphate by mixing in a flask distilled water with nearly twice its weight of crystals of pure zinc sulphate, and adding zinc oxide in proportion of about 2 per cent. by weight of the zinc sulphate crystals to neutralise any free acid. The crystals should be dissolved with the aid of gentle heat, but the temperature to which the solution is raised should not exceed 30 degs. C. Mercurous sulphate treated as described in (3) should be added in the proportion of about 12 per cent. by weight of the zinc sulphate crystals to neutralise any free zinc oxide remaining, and the solution filtered, while still warm, into a stock bottle. Crystals should form as it cools.
“5. The Mercurous Sulphate and Zinc Sulphate Paste.—Mix the washed murcurous sulphate with the zinc sulphate solution, adding sufficient crystals of zino sulphate from the stock bottle to ensure saturation, and a small quantity of pure mercury. Shake these up well together to form a paste of the consistency of cream.
Heat the paste, but not above a temperature of 30 degs. C. Keep the paste for an hour at this temperature, agitating it from time to time, then allow it to cool; continue to shake it occasionally while it is cooling. Crystals of zinc sulphate should then be distinctly visible, and should be distributed throughout the mass ; if this is not the case, add more crystals from the stock bottle, and repeat the
This method ensures the formation of a saturated solution of zinc and mercurous sulphates in water.
“To set up the Cell. “The cell may conveniently be set up in a small testtube of about 2 centimetres diameter, and 4 or 5 centimetres deep. . Place the mercury in the bottom of this tube, filling it to a depth of, say, .5 centimetre. Cut a cork about .5 centimetre thick to fit the tube; at one side of the cork bore a hole through which the zinc rod can pass tightly; at the other side bore another hole for the glass tube which covers the platinum wire; at the edge of the cork out a nick through which the air can pass when the cork is pushed into the tube. Wash the cork thoroughly with warm water, and leave it to soak in water for some hours before use. Pass the zinc rod about 1 centimetre through the cork.
Contact is made with the mercury by means of a platinum wire about No. 22 gauge. This is protected from contact with the other materials of the cell by being sealed into a glass tube. The ends of the wire project from the ends of the tube; one end forms the terminal; the other end and a portion of the glass tube dip into the mercury.
“ Clean the glass tube and platinum wire carefully, then heat the exposed end of the platinum red hot, and insert it in the mercury in the test tube, taking care that the whole of the exposed platinum is covered.
“Shake up the paste and introduce it without contact with the upper parts of the walls of the test tube, filling the tube above the mercury to a depth of rather more than one centimetre.
“Then insert the cork and zinc rod, passing the glass tube through the hole prepared for it. Push the cork gently down until its lower surface is nearly in contact with the liquid. The air will thus be nearly all expelled, and the cell should be left in this condition for at least 24 hours before sealing, which should be done as follows:
Melt some marine glue until it is fluid enough to pour by its own weight, and pour it into the test tube above the cork, using sufficient to cover completely the zino and soldering. The glass tube containing the platinum wire should project some way above the top of the marine glue.
“ The cell may be sealed in a more permanent manner by coating the marine glue, when it is set, with a solution of sodium silicate, and leaving it to harden. The cell thus set up may be mounted in any desirable
It is convenient to arrange the mounting so that the cell may be immersed in a water bath up to the level of, say, the upper surface of the cork. Its temperature can then be determined more accurately than is possible when the cell is in air.
In using the cell, sudden variations of temperature should, as far as possible, be avoided.
“The form of the vessel containing the cell may be varied. In the H form, the zinc is replaced by an amalgam of 10 parts by weight of zinc to 90 of mercury. The other materials should be prepared as already de scribed. Contact is made with the amalgam in one leg of the cell, and with the mercury in the other, by means of platinum wires sealed through the glass.”
A Clark's cell, made up according to the foregoing instructions, is represented in section by Fig. 17, which is approximately full size.
The voltage of this cell is very constant at a fixed temperature, but varies with a corresponding variation in temperature, the formula for calculating its E.M.F. at a given temperature being :
1 454 [1 - .00077 (1° -15°) ) volts. Modifications of the Standard Cell.Clark's standard cell, as specified by the Board of Trade, is almost universally adopted as a standard of electromotive force, but
there are, nevertheless, several modi
fied forms both this and other well-known types of primary battery which possess attendant advantages and disadvantages, rendering them more or less suitable, according to the local conditions governing the
in which their employment as standards is required. It is of these modifications, and their several merits and demerits, that I propose treating in the next few paragraphs, and shall commence by dealing with the larger section, viz., the Modifications of Clark's Standard Cell.