The cell may conveniently be set up in a small testtube of about 2 cm. diameter and 6 or 7 cm. deep. Place the mercury in the bottom of this tube, filling it to a depth of, say, 15 cm. Cut a cork about o'5 cm. 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. Pass the zinc rod about 1 cm. through the cork. 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 mercurous sulphate 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 2 cm. 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 twenty-four 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 should project above the top of the marine glue. The cell thus set up may be mounted in any desired 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. Fig. xliii gives a drawing of the cell thus set up. The cell thus prepared should have, at a temperature of 15° C., an E.M.F. of 1'434 volts. The E.M.F. decreases as the temperature rises by 00077 of its value per 1° C., so that FIG. xliii. Pt Zn Cork Zn.SO, with at to the E.M.F. is 1'434 (100077(-15)} If the cell is carefully set up in accordance with these instructions, its E.M.F. at the end of a Marine glue week or so should be within 2 or 3 in 10,000 of this value. Some cells shew considerable changes of E.M.F. when first made. This is usually due to one of two causes: either the solution is acid (in this case the free acid attacks the zinc, and the evil cures itself), or there is zinc oxide in the solution. In this case mercurous oxide is formed, and may be deposited as a grey powder on the zinc. The E.M.F. falls greatly, and remains too low. Neither of these defects should be taken to use neutral zinc sulphate, Paste of Mercury present if care has been free from zinc oxide. The cell in the form just described1 is not suitable for Το The cell may be set up in various other forms. For standard purposes the H form devised by Lord Rayleigh is probably best. secure portability, Professor Carhart introduces between the mercury and the paste a thin disc of cork, which fits the tube tightly; the cork must be well washed with warm water and left to soak in zinc sulphate solution before being used. Thus the mercury cannot become impure through contact with the zinc. Professor Carhart also coats the top of the marine glue with a thin layer of silicate of soda, which forms a hard and lasting glaze. In the cells sent out from the Berlin Reichsanstalt the positive pole is a piece of well-amalgamated platinum foil, which is surrounded by the mercurous sulphate paste. This is enclosed in a small porous pot. The outer vessel surrounding the porous pet contains only the zinc and zinc sulphate solution. zinc takes the form of a solid rod of amalgam, which is bent to an L shape. The vertical part of the rod is surrounded by a glass tube, the horizontal part, which alone is effective, is imbedded in the crystals of zinc sulphate. The use as a source of current. E.M.F., though in many cases it may conveniently be employed to measure a current in the manner described in (3) below. It is intended as a standard of (2) To use the Clark Cell as a Standard of E.M.F. Poggendorff's method of comparing electromotive forces has been described in § 80. If one of the two cells employed be a Clark's standard, any other E.M.F. can be compared with this. For many purposes it is desirable to use higher resistances than can be conveniently employed on a stretched wire bridge, and then the following method may be adopted :Connect up in series two resistance boxes A B, C D (fig. xliv) with a suitable battery. Let us suppose that we can take 10,000 ohms out of each box. The E.M.F. of the battery will depend on the value of the E.M.F. to be measured. If this be comparable with the E.M.F. of a Clark's cell, two Leclanché cells will be convenient. The boxes are to be used in such a way that the total resistance in circuit remains constant, and equal to say 10,000 ohms. Thus, if 4,500 be out in one box, 5,500 will be out in the other; and if an additional plug, say 5 ohms, is inserted in the first, the plug of the same value is taken out of the second. The connexions are made as in fig. 76, the resistance from A to P in that figure being represented by one box, AB, that from P to B by the second box, CD. It is desirable to put a high resistance in with the galvanometer when commencing the experiment. If increased sensitiveness is PP required, the resistance can be reduced or removed as the state of balance is approached. Fig. xliv shews the practical arrangement of the connexions for comparing two unequal electromotive forces E, E1. If R be the resistance in one box, R' that in the second when there is no current through the galvanometer and the battery E is in circuit, and if R1, R' are the corresponding resistances for E,, then If the two electromotive forces are nearly equal, as, for example, those of two Clark cells, a better method is to connect the two in opposite directions and compare the difference of their electromotive forces with the electromotive force of one of them or of a third standard cell. (3) To use a Clark Cell to Measure a Current. This is effected by passing the current through a wire of known resistance, and comparing the E.M.F. between the ends of the wire by the potentiometer method with that of the Clark cell. Let c be the value of the current, R the resistance employed, E the E.M.F. of the Clark. The potential difference between the ends of the wire is CR, and if R1, R2 be the potentiometer readings, as described in § 80, or in (2) above, we have The resistance R should be so chosen that the E.M.F. CR may be comparable with that of the Clark. Moreover, since the current heats the wire, and the resistance changes with temperature, the size and material of the wire should be such as to make this change inappreciable. In some cases it is more convenient and simpler to use the current to be measured as the main current of the potentiometer. In this case the current is passed through the potentiometer from A to B, the positive pole of the cell is connected to A, and the negative pole through the galvanometer to a point P on the wire such that no current passes through the galvanometer. When this is the case, if R be the resistance of A P, C the current, and E the E.M.F. of the Clark cell, we have E = CR; F .. C= R Figs. xlv and xlvi shew the connections. The stretched wire may, of course, be used instead of the resistance boxes in fig. xlvi. The converse of this method is the one employed for determining absolutely the E.M.F. of a Clark or any other cell, for if in the above A FIG. xlv R B equation cand R are known absolutely, E is given in absolute c can be measured in various ways, e.g. as in §71, units. by the use of a tangent galvanometer, though this would probably not be the best method to adopt, or as in the next section, by the electrolysis of silver. X. The Silver Voltameter. We have discussed in § 72 the method of determining the reduction factor of a galvanometer, and thereby mea |