APPENDIX C

OFFICIAL SPECIFICATION FOR THE PREPARATION OF THE CLARK CELL

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 be first 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 with it 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 ("pure recrystallized") 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 the proportion of about 2 per cent by weight of the zinc sulphate crystals to neutralize 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° 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 neutralize 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 mercurous sulphate with the zinc sulphate solution, adding sufficient crystals of zinc 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 consistence of cream. Heat the paste, but not above a temperature of 30° 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 whole process.

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 test tube 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 05 centimetre. Cut a cork about 0.5 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 cut 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 part of the walls of the test tube, filling the tube above the mercury to a depth of rather more than 1 centi

metre.

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 zinc 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 manner. 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 described. 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.

PROBLEMS AND EXERCISES

QUESTIONS ON CHAPTER I

1. In what respects does an electrified body differ from a non-electrified body?

2. Name some of the different methods of producing electrification.

3. A body is charged so feebly that its electrification will not perceptibly move the leaves of a gold-leaf electroscope. Can you suggest any means of ascertaining whether the charge of the body is positive or negative?

4. How would you prove that the production of a positive charge is accompanied by the production of an equal negative charge?

5. Describe an experiment to prove that moistened thread conducts electricity better than dry thread.

6. Why do we regard the two electric charges produced simultaneously by rubbing two bodies together as being of opposite kinds?

Can you sug

7. Explain the action of the electrophorus. gest any means for accomplishing by a rotatory motion the operations of lifting up and down the cover of the instrument so as to obtain a continuous supply instead of an intermittent one?

8. Describe the state of the medium between two oppositely charged bodies, and state how you would determine the direction of the lines of force at any point.

9. Explain the Torsion Balance, and how it can be used to investigate the laws of the distribution of electricity.

10. Describe what takes place as an electrified conducting ball is made to approach a large conducting surface. Show by diagram the direction and relative number of the lines of force.

11. Two small balls are charged respectively with +24 and -8 units of electricity. With what force will they attract one another when placed at a distance of 4 centimetres from one another? Ans. 12 dynes.

12. If these two balls are then made to touch for an instant and then put back in their former positions, with what force will they act on each other?

Ans. They will repel one another with a force of 4 dynes. 13. Enumerate the essential parts of an influence machine; and explain how they operate to produce electrification.

14. Take the diagrammatic representation of the Wimshurst machine (Fig. 40) and fill in the lines of electric force, showing their direction and relative number.

15. Explain the action of the Leyden jar by the consideration of electric displacement.

16. Describe four different ways of electrifying a tourmaline crystal.

17. Zinc filings are sifted through a sieve made of copper wire upon an insulated zinc plate joined by a wire to an electroscope. What will be observed ?

18. Explain the principle of an air-condenser; and state why it is that the two oppositely charged plates show less signs of electrification when placed near together than when drawn apart from one another.

19. There are four Leyden jars A, B, C, and D, of which A, B, and D are of glass, C of guttapercha. A, B, and C are of the same size, D being just twice as tall and twice as wide as the others. A, C, and D are of the same thickness of material, but B is made of glass only half as thick as A or D. Compare their capacities.

Ans. Take capacity of A as 1; that of B will be 2; that of C will be ; and that of D will be 4.

20. How would you show that a bar made half of zinc and half of copper is capable of producing electrification?

21. How would you prove that there is no electrification within a closed conductor?

22. What prevents the charge of a body from escaping away at its surface?

23. Explain the action of Hamilton's mill.

24. Two brass balls mounted on glass stems are placed half an inch apart. One of them is gradually charged by a machine until a spark passes between the two balls. State exactly what happened in the other brass ball and in the intervening air up to the moment of the appearance of the spark.