A to E can be made precisely two volts, and the current consequently two milliamperes. The procedure, then, is to set the switches at the value appropriate to the standard cell (for a Clark cell of 1*434 volts, 717 ohms); adjust till no deflection of the galvanometer is obtained; then balance the experimental cell instead, and the switch reading multiplied by o'002 will give the required voltage. This method is especially convenient when a Weston cell of zero temperature coefficient is used as standard, and the resistance between A and E can, by a modified construction of the box, be adjusted to 101'9, 1019, or 10190 ohms. Sliding Contact.-In box potentiometers this is an ordinary contact lever, and presents no difficulties; but with the slidewire the contact-maker has usually to be pressed down by hand. It consequently gets warmed, and may set up thermoelectromotive force. The best way to avoid this is to have two parallel stretched wires of the same metal, and a short sliding bridge to connect them. The main current flows through one; the other is connected to the galvanometer only. The symmetry of the arrangement makes the thermo-electric effects neutralise. It is also usually preferable to arrange for the slides to make contact always, and use a separate key to put the galvanometer into circuit. In many complete potentiometers a special switch is provided by which any one of several cells can be put into action. This allows of measuring them quickly one after the other, without the trouble of unscrewing wires, and the risk of thermoelectric errors in doing so. Indicating Instrument.-Either a galvanometer or capillary electrometer may be used. The former is the more sensitive and convenient, but is influenced by the resistance of the cell to be measured. If that resistance is very high, the galvanometer becomes less useful than the electrometer, which—for small potential differences-behaves as a static instrument. A good galvanometer of moderately high resistance should indicate 10 amperes, and so, even if the internal resistance of the cell being tested is 10,000 ohms, will allow of its electromotive force being measured to 10 volts. There are very few chemical combinations whose electromotive force will remain steady to that extent. The capillary electrometer consists essentially of two electrodes of mercury with sulphuric acid (or other electrolyte) between them, the contact of acid and metal being made, on one side, in a capillary tube. A good form is shown in Fig. 48. The wide tube b is filled with mercury and connected by a platinum wire, d, with one pole. The mercury extends to a in the capillary tube, and on account of the surface tension of the mercury a is below the level of the liquid in the wide tube. The other mass of mercury is contained in the bulb e, connection being made by a wire, g, sealed through a glass ее FIG. 48. еееее tube. Electrification of mercury causes its surface-tension relatively to an electrolyte to change. Hence if an electromotive force be applied to the instrument, the mercury in a will rise or fall. The capillary mercury must not be made positive by more than one or two centivolts, but may be made considerably negative. In using the instrument with a potentiometer the voltage applied to it should be kept down as much as possible. The mercury meniscus may be observed with a microscope of moderate magnifying power, provided with a short ocular scale. It is then capable of indicating down to about 10 volts. The electromotive force of any complete voltaic combination can be measured by connecting its electrodes to a voltmeter or potentiometer, and that whether current is flowing through it or not. E.g., the potential differences between the terminals of an accumulator during charge and discharge can. be studied in this manner. It must be borne in mind that if current is flowing, a certain difference of potential will occur between the poles in consequence of the resistance of the cell. If this be R, and the strength of current (excluding any required by the voltmeter) i, then, in accordance with Ohm's law, the potential of the anode must be higher than that of the cathode by Ri, irrespective of any electromotive force in the cell. In order to obtain the true electromotive force when current is flowing, the resistance must be measured, and this correction applied. B.-MEASUREMENT OF SINGLE POTENTIAL DIFFERENCES. The same methods are applicable for determining single electrode potentials, if the electrode to be measured is combined with some standard electrode so as to make up a voltaic combination. Thus, the electrode is very convenient for use with the lead accumulator. If some mercury be placed in a glass tube, covered with mercurous sulphate and dilute sulphuric acid, and the whole sunk in the acid of an accumulator, we may measure the potential difference between either plate and an insulated wire dipping in the mercury of this standard electrode. In this way any electrode potential may be measured if only we know the absolute potential of any one standard. Absolute potentials have not, so far, been determined by any means quite free from objection, but certain approximate values have been obtained by the methods described above (p. 194), and may be accepted provisionally as correct. In general an electromotive force is set up at the contact of two solutions, and consequently where the liquid of the standard electrode touches that of the electrode to be measured. For an accurate measurement of electrode potentials it is therefore necessary to allow for this, and as the correction, though small, is uncertain, it is important to choose standard electrodes that give rise to as little electromotive force of contact between solutions as possible. The most important standard electrodes in use are the following: Calomel Electrode.—This consists of mercury covered with mercurous chloride as depolariser, and immersed in a solution of potassium chloride, which may be either normal or decinormal. Potassium chloride is in most cases exceptionally favourable as to electromotive force of contact with other solutions, so the calomel electrode is usually a good one to use. It is also easy to set up, and the materials can be obtained pure without difficulty. The potential of the normal electrode is usually taken as +0'5600 (metal positive to solution) at 18°, increasing o'0006 per degree above; that of the decinormal as +06130 at 18°, temperature coefficient 0'0008. A convenient construction of the electrode is shown in Fig. 49. The mercury at the bottom of the tube is covered, about FIG. 49. 1 cm. deep, with calomel that has been several times washed with KCl solution, and shaken up with mercury to remove any mercuric chloride; the rest of the tube contains the KCl solution. It is placed with its point immersed in a beaker of the same solution, and the bent tube filled by blowing through the side tube on the right. After use a few drops of the liquid in the bent tube may be blown out, and the remainder sucked back. The other electrode whose potential is required may be made in similar form, and also allowed to dip in the beaker of solution. Contact is made with the mercury of the normal electrode by a platinum wire sealed through a glass tube; or, as in Clark and cadmium cells, liquid mercury may be dispensed with, an amalgamated platinum sheet being immersed in the calomel paste instead. care, calomel electrodes agree millivolt. When made up with due amongst themselves to about The decinormal electrode is stated by Richards1 to be more constant and less disturbed by vibration than the normal. According to Ostwald and Luther,2 this is not the case if the electrode be properly made up; and the normal electrode has of course the advantage of a lower resistance. The merturous sulphate electrode referred to above, for use with accumulators, is similarly constructed. Its potential with equivalent normal acid is + 0°956 volt. For alkaline solutions the combination Hg: HgO: n.NaOH may be used. Potential +0'387. Silver, coated with silver chloride or bromide, and immersed in the corresponding potassium solution, may be used instead of mercury.3 Besides electrodes of the calomel type, the most important standard that has been used is the hydrogen electrode. This, as used by Wilsmore, is shown in Fig. 50; the form is convenient and suitable for accurate work, and may be adopted for electrodes of other gases as well. The gas is led in through a Richardson wash-flask K; this is filled with the same solution as is to be used in contact with the electrode, in order that the gas should be saturated with the vapour of that solution, and șo not cause any change in concentration as it bubbles past the electrode. From the wash-flask it enters the bottom of tube A. This is closed at the top by a rubber stopper, through which passes a glass tube carrying the metal sheet which is to act as electrode; the latter may be palladium, or well platinised 1 Zeitschr. phys. Chem., 24. 37 (1897). 2 Physiko-Chemischen Messungen (Leipzig: Engelmann), 2nd ed., 1902, q.v. for further details of potential measurements. 3 For details see Jahn, Zeitschr. phys. Chem. 33. 555 (1900). + Zeitschr. phys. Chem., 35. 296 (1900). |