Abstract
Physical Society, October 27.—Prof. J. Perry, F.R.S., vice-president, in the chair.—Mr. E. C. Rimington read a paper “On the Behaviour of an Air-Core Transformer when the Frequency is Below a certain Critical Value.” Taking the ordinary differential equations for two circuits having self and mutual induction, and assuming sinusoidal E.M.F.'s and constant coefficients, the author shows that although the difference of phase between the primary P.D. and primary current is always diminished on closing the secondary circuit, yet under certain circumstances this closing increases the impedance of the primary. With constant P.D. this means that closing the secondary decreases the primary current, a phenomenon not usually observed. The critical conditions necessary for increased impedance are fully worked out in the paper, as well as those under which this increase becomes a maximum. In the case of two identical coils with no magnetic leakage, the critical value of α ( where p = 2π times the frequency, L the inductance of the primary, and r1 its resistance) is √2, whilst that to give maximum impedance is (. The maximum increase possible is 15½ per cent. The corresponding values are given for various amounts of magnetic leakage in tabular form, and curves were exhibited at the meeting showing how the impedance, current, power, and magnetising effect vary for different values of α. To test his conclusions the author made experiments on two coils close together, the observed increase in impedance amounting to 3˙2 percent. In addition to the analytical investigation, the subject is treated geometrically at considerable length. Prof. Minchin showed that the impedances might be represented by two hyperbolas, having p2 as abcissæ and the squares of the impedances as ordinates. These could be readily constructed from the data given. A line representing the primary inductance drawn on the same diagram intersects one hyperbola, showing that the impedance has always a maximum value. By a simple construction the phase angle between the primary and secondary currents could be determined for any given conditions. Dr. Sumpner observed that increased impedance on closing the secondary necessarily meant a decrease in the lag of the primary current behind the primary P.D. Mr. Blakesley was pleased to see the geometrical method of such service, and thought it much simpler than the analytical one. The reason why increased impedance on closing the secondary of ordinary transformers had not been noticed was because their lag angles were very large. In a figure published some years ago to represent the actions of transformers, the angles he had chosen were such as would make the primary impedance increase on closing the secondary. Giving an expression connecting the primary currents on open and closed secondary respectively, he now showed that to get increased impedance, the sum of the lag angles in primary and secondary must exceed 90°. To get large power in the secondary the primary lag should be nearly 90°, and the secondary about 45°. He also pointed out that some of the figures in the paper might be simplified considerably. Prof. Perry said he had long had the impression that if a sufficiently small current were taken from the secondary, increased impedance would be observable in all cases, and he quoted some numbers he had given in the Phil. Mag. for 1891, showing a decided increase. Mr. Rimington, in reply, said he was not aware that the effect he had now brought forward had been observed previously. The result was completely worked out analytically before using geometrical methods.—Mr. W. B. Croft showed “Two lecture-room experiments.” One, on “The Rings and Brushes in Crystals,” was performed by very simple apparatus in two ways. In the first, a bundle of glass plates was used as a polariser, and a Nicol prism as analyser. When a Nicol could not be conveniently obtained, a glass plate could be used as a reflecting analyser. For a convergent system two glass card-counters were used, the crystal being placed between them. Very good results were produced by this simple apparatus. In the second arrangement the crystal was placed on the eye-piece of a microscope (whose objective was removed), and covered by a tourmaline. On reflecting light up the tube by means of a piece of glass held at the proper angle excellent results were obtained. Another experiment, on “Electric Radiation in Copper Filings,” was similar to those described by Dr. Dawson Turner at the Edinburgh meeting of the British Association. A battery, galvanometer, and glass tube containing copper filings were joined in series. Under ordinary circumstances no current passed, but immediately an electric spark was produced by an electric machine many feet away, the galvanometer was violently deflected, and remained so until the tube was tapped. On trying different materials, aluminium and copper seemed about equal, but iron not so good; carbon allowed the current to pass always. Prof. Minchin said the phenomena were strikingly like those exhibited by his “impulsion cells,” for the moment a spark passed, even at a distance of 130 feet, they became sensitive to light. Very minute sparks were capable of producing the change, but by adding capacity to the sparking circuit the effect could be greatly modified. Replying to a question from Mr. Rimington, he said the change was due to electromagnetic vibrations, and not to light emitted by the sparks. Mr. Blakesley inquired if lengthening the sparks produced greater effect on the copper filings. Mr. Lucas asked if the resistance of a tube ever became infinite again if left for a long time. In reply, Mr. Croft said the current sometimes passed before the spark actually occurred between the knobs. He had not left tubes for very long, and had not found the resistance reappear without tapping.
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Societies and Academies. Nature 49, 46–48 (1893). https://doi.org/10.1038/049046b0
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DOI: https://doi.org/10.1038/049046b0