SUBJECTS electromotive force

A second source of standard free energies comes from the measurement of the electromotive force of a galvanic cell. Electrochemistry is the subject of other articles (A2.4 and B1.28). so only the basics of a reversible chemical cell will be presented here. For example, consider the cell conventionally written as... 

The changes, however, are both numerous and significant. First of all, there is a change in the organization of the subject matter. For example, material formerly contained in the section entitled Analytical Chemistry is now grouped by operational categories spectroscopy electrolytes, electromotive force, and chemical equilibrium and practical laboratory information. Polymers, rubbers, fats, oils, and waxes constitute a large independent section. 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

Thus, in the decades after 1880, many physicists and some chemists especially interested in mathematical approaches to their subject matter turned their attention to the calculation of free energies in both physical and chemical processes, with renewed attention to heat capacities, electromotive force, and values for molecular weights and molecular concentrations. Studies of reaction rates became more and more common, following up on path-breaking work by the physicist L. Wilhelmy and the chemist Vernon Harcourt.65... 

The Peltier effect has been shown to be reversible, and is. therefore subject to Le Chatelier s principle, like all reversible processes. Thus, when the boundary between two metals is heated, an electromotive force must be produced. The direction of this E.M.F. must be such as to oppose a current which would produce a positive Peltier effect (that is, a heating effect) at the boundary. Thus a current will flow in a closed circuit made up of two different metals when the two boundaries are at different temperatures. No current will flow when the two boundaries are at the same temperature, even when the temperature at other parts of the circuit is not uniform. In the first case the E.M.F. in the circuit is E = — E2+E — E2, where E and 2... 

Resistance of Indicating Instruments.— When operated at the highest safe working temperatures most base-metal couples develop a maximum electromotive force of less than 50 to 70 millivolts and the LeChatelier couple an electromotive force of about 16 millivolts. In order to measure such small electromotive forces accurately a very sensitive indicator or millivoltmeter is required. On the other hand the instrument must be robust and able to withstand rough handling to which it is necessarily more or less subjected. These conditions of mechanical robustness and of high accuracy as a pyrometer indicator are difficult to satisfy. 

Until now it was assumed that interaction between currents is absent, i.e. all currents induced in a conducting nonuniform medium, regardless of the distance from the source, are shifted in phase by 90°. For this reason electromotive forces induced in measuring coils of a probe are in phase with each other, and they are added and subtracted in the same way as scalars. If only one component of the electromotive force, for instance the quadrature component, is measnred it is subjected to the same operations as scalars, regardless of whether currents are shifted in phase by 90°, or the internal skin effect manifests itself and due to it at every point of a medium there are both quadrature and inphase components of the induced current. 

However, in more complicated cases, when measured electromotive forces are subjected to the influence of parameters of the borehole and the invasion zone, interpretation cannot be performed without additional information. 

For sufficiently large times the main role in asymptotic formulae is played by the first term, and as follows from a physical point of view it does not depend on parameters of the borehole and the invasion zone and coincides with the leading term of expansion at the late stage in a uniform medium with the formation resistivity. Thus, regardless of the probe length there is always a moment, starting from which the measured field, as well as the electromotive force, are not practically subjected to the influence of induced currents in the borehole and in the invasion zone. 

For electromotive force measurements there are many suitable electrodes, and only the most novel features can be summarised here. The glass electrode is generally usable in these solvents. It has been found that in solutions of hydrochloric acid in anhydrous acetic acid, the glass electrode is subject to acid errors of as much as 70 mV relative to the chloranil electrode, owing to the incorporation of chloride ions in the surface gel layer of the glass. ° Since ethylenediamine reacts with calomel but not with mercury (II) chloride, a suitable reference electrode for this solvent can be constructed of a mercury pool in contact with ethylenediamine saturated with respect to both HgClg and LiCl. °... 

The Status of the Hydrogen Electrode. Probably no area of electrochemistry is more greatly neglected in current texts than the history of the choice of the hydrogen electrode as the reference standard for electromotive force measurements. Since all tables of potentials of oxidation-reduction half-reactions are based on the half-cell reaction 35H2=H +e , it would seem that the selection of this reaction as the standard should warrant more attention. If the selection is treated at all, it is usually dismissed as an arbitrary choice, which it is, with no reference made to the people and events involved in establishing this fundamental reference point for the EMF scale. One possible exception may be noted ( ). The referenced edition of this work is perhaps the best previously existing source on this topic. However, the subsequent edition omits the subject entirely. 

The standards of electromotive force available up to the present time did not come up to the ideal which Mr. Weston had set before him, even the best of these, the Clark cell, being subject to a variation of 0.077 per degree centigrade. 

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