Abstract
LONDON.
Physical Society, June 10.-Mr. Walter Baily, Vice-President, in the chair.-Dr. Gladstone read a paper on some points connected with the ¦ electromotive force of secondary batteries, by himself and Mr. W. Hibbert. The communication includes replies to certain questions raised by M. Darrieus in a paper read before the Société Internationale des Electriciens on May 4, 1892; and to the views expressed by Prof. Armstrong and Mr. Robertson in the discussion on a paper by the present authors read before the Institution of Electrical Engineers, on May 12 and 19. It also contains an account of their recent experiments on the subject. M. Darrieus agrees with Prof. Armstrong and Mr. Robertson that the large E.M.F. immediately after charge is due to persulphuric acid, and opposes the ordinary theory that the ultimate product of discharge is lead sulphate at both plates, so far as the positive plate is concerned. The authors attribute the finding of large quantities of lead oxide by M. Darrieus to difficulties in analysis, for it is not easy to imagine that oxide of lead could remain as such in presence of sulphuric acid. They have also shown that the changes of E. M.F. during charge and discharge coincide fairly well with those obtained by putting Pb and PbO2 plates in different strengths of acid, and conclude “that the changes of E.M.F.... depend on the strength of the acid that is against the working surfaces of the plates.” Prof. Armstrong and Mr. Robertson disagree with the authors' views, and suppose that the sulphuric acid used was ' contaminated with soluble peroxides; and they also believe that H2SO4 itself takes part in the reactions. As regards the first objection, the authors see no reason why the traces of soluble peroxide (if any) on the plates should always vary in amount with the strength of the fresh acid in which the plates were dipped. The second point they leave an open question. In reply to the criticism on the summation of the two curves obtained respectively with two lead plates and two lead peroxide plates in acids of different strengths, they point out that the resulting curve coincides both in shape and magnitude with that determined when a Pb and a PbO2 plate were placed in different strengths of acid. Whilst admitting the possibility of the lead supports having some influence on the result, they cannot conceive that such large and uniform differences as those given in their paper can be due to accidental operations of local action. To show that the increase of E.M.F. does not depend on the presence or absence of persulphuric acid, the authors have tested the E.M.F. of a Pb and a PbO2 plate, free from soluble oxides, in sulphuric acid of 15 per cent, strength, a porous diaphragm being between the plates. The E.M.F. was J'945 volts. After adding I per cent, of persulphate of potassium to the liquid surrounding the PbO2 plate, the E.M.F. was unaltered; whilst putting the Pb plate in the same liquid only reduced the E M.F. to i'934- Experiments had also been made on cells with phosphoric acid of different strengths, instead of sulphuric acid. Changing the density from 1 05 to 1'5, raised the E.M.F. 0T76 volt, whilst calculations from Lord Kelvin's law gave o'i7i volt. In this case they consider that no acid analogous to persulphuric acid could be present. They also find that the effects of charging and repose on the E.M.F. of phosphoric acid cells are quite analogous to those obtained with sulphuric acid. The researches are being extended chiefly on the thermochemical side. Prof. Ayrton thought there was no question that the strength of acid had much to do with the changes of E.M.F. The point at issue, he considered, was whether the changes were direct effects of the strength of acid, or due to secondary actions brought about by alterations in strength. Mr. E. W. Smith said Mr. Robertson and himself were repeating the author's experiments with two PbO2 plates without any grid. They had obtained results analogous to those mentioned in the paper, but the true explanation of the effects was still to seek. Mr. W. Hibbert contended that the soluble oxides referred to by Prof. Armstrong and Mr. Robertson were not present in their experiments. They had also proved that changes in acid strength altered the E.M.F., whilst presence of persulphuric acid did not. Dr. Gladstone, in reply, said they also were making experiments without grids, but had not made sufficient progress to discuss them at present. Mr. Hibbert and himself believed the effects of local action inconsiderable, whilst Messrs. Armstrong and Robertson thought them very important. He hoped that ere long the points would be settled conclusively.--A paper on workshop ballistic and other shielded galvanometers, by Prof. W. E. Ayrton, F.R.S., and Mr. T. Mather, was read by Prof. Ayrton. The galvanometers described were of the type having movable coils and fixed magnets, the advantages of which are well known. In designing the ballistic instruments, their aim had been to obtain sensibility and portability, combined with being screened from external influences, for it was often desirable to measure the magnetic fluxes and fields in dynamos by apparatus near the machines. One of the improvements adopted was the narrow coil described in a paper "On the Shape of Movable Coils, c,"read before the Society in 1890. Such coils are particularly advantageous for ballistic instruments, for not only can greater swings be obtained by the discharge of a given quantity of electricity through such a coil than with ordinary shaped coils when the periodic times are the same, but even when the same control is used, the same length of wire in the coil, and suspended in the same field, the narrow coil is more sensitive to discharges than coils of any other shape. Another improvement was the use of phosphor bronze strip for the suspensions instead of round wire. For a given tensile strength, both the control and the subpermanent set could be diminished by using strip. In February 1888 the authors made a d'Arsonval of the ordinary type as a ballistic instrument, and found that although it was suitable for comparing condensers, yet for induction measurements the damping was excessive unless the resistance in the circuit was very large. This greatly reduced the sensitiveness. In 1890 they tried one of Carpentier's milliamperemeters as a ballistic instrument, but found it insensitive. A narrow coil instrument made in the same year was found to be sensitive for currents; but as the coil was wound on copper to get damping, it was not suitable for ballistic work. In January 1892 a somewhat similar instrument was constructed for ballistic purposes, and was found very sensitive and convenient. Although the coil had only a resistance of 13 ohms, one microcoulomb gave a swing of 170 divisions on a scale 2000 divisions distant, the periodic time being 2*7 seconds. The instrument could be used near electromagnets or dynamos, and was so sensitive that for ordinary induction measurements very large resistances can be put in series with it, thus reducing the damping to a very small amount. On the other hand, the coil could be brought to rest immediately by a short circuit key. It had the further advantage that it was not necessary to redetermine its constant every time it was used. The chief disadvantage of such instruments was the variable damping on closed circuits of different resistances. This could, however, be overcome by arranging shunts and resistances so that the external resistance between the galvanometer terminals was the same for all sensibilities. A portable ballistic instrument, intended for workshop use, was next described. This had a narrow coil and a pointer moving over a dial whose whole circumference was divided into 200 parts. The instrument had been designed to give a complete revolution for a reversal of a flux of two million C. G. S. lines, but the pointer could turn through two or more revolutions. To test strong fields a test coil with a total area of 10,000 square centimetres is used, and has a trigger arrangement for suddenly twisting it through two right angles. The instrument then reads off directly the strength of field in C. G.S. lines. To vary the sensitiveness in known proportions, resistances are employed. Referring to the improvements made in movable coil instruments since January 1890, when a paper on "Galvanometers "was read before the Society by Dr. Sumpner and the present authors, Prof. Ayrton said Mr. Crompton had greatly increased the sensitiveness of Carpentier's instruments by suspending the coils with phosphor-bronze strip. Mr. Paul had brought out a narrow-coil instrument which combined the advantages of portability, dead-beatness, quickness, and sensibility. Specimens of ihese instruments were exhibited. The narrow coils are inclosed in silver tubes, which serve to damp the oscillations. Such a coil is suspended within a brass tube which also forms the mirror chamber, and slides down between the poles of a circular magnet fixed to the base. To clamp the coil, a plug mounted on a slotted spring passes through a hole in the brass tube. A tube can be taken out and replaced by another containing a coil of different resistance in a few seconds. An instrument of this kind, with a coil of 300 ohms, gave 95 divisions per microampere, and the damping on open circuit was such that any swing was -fa of the previous one. On comparing recent instruments with those mentioned in the paper on galvanometers above referred to, a distinct improvement is apparent, for their sensitiveness is, for the same resistance and periodic time, as great as that of Thomson instruments. Prof. Perry remarked that the forces dealt with were extremely small. Mr. Swinburne thought that ballistic galvanometers might be regarded as instruments indicating the time integral of E.M. F. rather than quantity. Illustrating his meaning by reference to dynamos, he said that if two machines arranged as dynamo and motor were joined by wires, then, if the armature of the dynamo were turned through any angle, that of the motor would move through the same angle, supposing friction, c., eliminated. Speaking of figures of merit, he pointed out that the power consumed was the important factor. Prof. S. P. Thompson inquired what was the longest period yet obtained with narrow-coil instruments. The decay of magnetism in large dynamos was so slow that very long periods were required. He himself had used a weighted coil for such measurements. He also wished to know why the figures of merit were expressed in terms of scale divisions on a scale at 2000 divisions distance, instead of in angular measure or in tangents. Mr. E. W. Smith asked what was the length of strip required to prevent permanent set when the deflection exceeded a revolution. Mr. A. P. Trotter thought that, in testing magnetic fluxes by the workshop ballistic instrument-, the test coil might be left in circuit instead of putting in another coil. He wished to know what error was introduced by the change of damping caused by the resistance of the circuit not being quite constant. In his reply, Prof. Ayrton said Mr. Boys had pointed out that the scientific way to lengthen period was not by weighting the coils or needles, but to weaken the control. Periods of 5 seconds had been obtained. At present it was not easy to obtain longer periods owing to difficulties in obtaining sufficiently thin strip, and to the magnetism of materials.
Article PDF
Rights and permissions
About this article
Cite this article
Societies and Academies. Nature 46, 214–216 (1892). https://doi.org/10.1038/046214b0
Issue Date:
DOI: https://doi.org/10.1038/046214b0