Abstract
LONDON. Physical Society, May 28.—Prof. W. E. Ayrton, Vice-President, in the chair.—Dr. S. P. Thompson read a note on transformers for electric distribution. In the simple algebraic treatment of the dynamo several assumptions approximately true for well-made machines are made use of. The author finds that a similar set of assumptions for transformers greatly simplifies the algebraic theory:—(1) The iron, copper, and insulation are assumed good. (2) The reaction of the secondary on the primary (other than that desired) is small; thus, if the primary be supposed to be supplied with constant mean current or constant mean potential difference, this is not to be altered by the current in the secondary. (3) No magnetic leakage; so that the coefficient of mutual induction is the geometric mean between their coefficients of self-induction. (4) The quantities of copper in the primary and secondary are to be equal. These assumptions are shown to be legitimate, and the ratios of the resistances, E.M.F.'s, currents, and coefficients of self-induction are expressed in terms of the ratio of the numbers of convolutions, which ratio is represented by p = S1/S2. From analogy with the dynamo it is shown that E2 = ωM/√R12+ωL12E1, where ω = 2πn, E1, and E2 the E.M.F.'s of the primary and secondary respectively, and R1 and L1 the resistance and self-induction of the primary coil. If R1 be negligible, the above reduces to E2 = ωM/√ω2L12=E1/p, since L1/L2 p2 and M = √L1L2. The latter part of the paper contains a general investigation of two neighbouring circuits both having self-induction, and it is shown that the effective resistance of the primary is increased, and the self-induction decreased by closing the secondary circuit. Mr. Kapp said the investigations assumed the coefficients of induction to be constants, and that the phases of current in primary and secondary were opposite. The former being by no means true, he asked, What values were to be taken? and he believes the phases of current are not opposite in ordinary transformers. Mr. Swinburne protested against the use of formulæ to calculate the inductions when the required data could be obtained much more accurately from Dr. Hopkinson's curves on magnetization of iron. He also thought the curve of sines did not nearly represent the current curve for ordinary machines. Mr. Bosanquet thought the effective magnetization of a transformer would be different from that of a dynamo, for, in the former, permanent magnetism was not utilized. In reply to Mr. Kapp and Mr. Swinburne the author pointed out that as the coefficients of induction enter in both numerator and denominator, it would not matter which set of values were taken if the resistance was small compared with ωL; and that self-induction tends to jooth out irregularities in the current curve. Prof. Ayrton described a method of regulating a series transformer devised by himself and colleague some two years ago, based on analogy with a compound dynamo. Referring to the variation of L with current, he sketched a curve connecting them, obtained by Mr. Sumpner at the Central Institution, and mentioned that the E.M.F. curve of a Ferrani dynamo is an exact sine curve. He believes problems involving alternating currents would be greatly simplified by using a new set of measurable quantities, such as will render the equations as simple as possible. At Prof. Thomps m's request, Prof. Ayrton exhibited a lecture experiment illustrating the action of transformers. The secondaries of two ordinary induction coils were joined in series through long fine wires, and an incandescent lamp placed in the primary circuit of one, lighted up on completing the primary of the other coil in which a battery was placed.—On magnetic torsion of iron wires, by Shelford Bidwell. This is an account of experiments made on the twisting produced by sending a current along magnetized iron wires, and the author shows that Wiedemann's explanation of these phenomena (by assuming a difference in molecular friction at the polar and lateral surfaces of magnetized molecules), is unsatisfactory. The wires were magnetized longitudinally by means of a solenoid in the axis of which the wires were suspended. To obtain consistent results it was found necessary to demagnetize the wire between the observations. This is done by reversed currents of gradually decreasing strength, and a simple arrangement of rheostat and commutator devised for this purpose was exhibited. Two sets of experiments were made, in one of which the current in wire or solenoid was kept constant whilst that in the other was varied. The amount of twisting does not increase continuously when the currents are increased, but attains a maximum when the inclination of the helix, representing the direction of magnetization, is inclined at about 33° to the axis of the wire. When the current in the solenoid was kept constant and that in the wire increased, permanent deflections remained on stopping the current. For small currents in the wire this deflection was diminished on starting the current, whilst stronger currents increased the deflection. For some intermediate value of the current, no change took place, and this value was dependent on the current in the solenoid. Experiments were shown illustrating these phenomena.
Article PDF
Rights and permissions
About this article
Cite this article
Societies and Academies . Nature 36, 142–144 (1887). https://doi.org/10.1038/036142b0
Issue Date:
DOI: https://doi.org/10.1038/036142b0