598 



NA TURE 



[April 



1903 



SOCIETIES AND ACADEMIES. 

 London. 



Royal Society. — "A New Form of Self-restoting Co- 

 herer." By Sir Oliver Lodge, F.R.S. Communicated 

 verbally March 12, received in manuscript March 18. 



The essential part of the receiving instrument now always 

 employed in the system of Hertzian telegraphy, which Dr. 

 Muirhead and the author had brought out and always now 

 employed, and which their assistant, Mr. E. E. Robinson, had 

 helped to work out, might be described as a development 

 of the mercury form of coherer described some years ago 

 by Lord Rayleigh, and again in a modified fashion by Mr. 

 Rollo Appleyard. In Lord Rayleigh's form this consisted 

 of a pool of mercury cut across with a paraffined knife, and 

 the two half pools connected to a battery and key. As soon 

 as the key was depressed so as to throw a few volts on to 

 the intervening film of oil, the electrostatic pressure seemed 

 to squeeze the oil out, and the pools of mercury became one. 

 The pressure exerted by a few volts on a film of barely soap- 

 bubble thickness is very considerable, and comparable to 

 a ton per square inch. 



Needle points dipping in oil and mercury were tried as 

 practical coherers, the points being pulled out electro- 

 magneticallv every time a signal arrived. Rotating forms 

 of contact for automatic decoherence were also tried in 

 various forms, and ultimately the method took the form 

 of a rotating sharp-edged steel wheel, about half an inch 

 in diameter, constantly touching a pool or column of 

 mercury on which was a thin layer of oil. No effective 

 contact occurs between the wheel and the mercury, not- 

 withstanding the immersion, because of the film of oil; 

 but the slightest difference of potential applied to the two, 

 even less than one volt, is sufficient to break the film down 

 and complete a circuit, which, however, the rotation ol the 

 wheel instantaneously breaks again. A spark is so 

 sudden that for its purposes the wheel is for the instant 

 virtually stationary, and yet the decohesion is so rapid 

 that signals can be received in very rapid succession. The 

 definiteness of the surfaces and of the intervening layer 

 make the instrument remarkably trustworthy, and the thin- 

 ness of the insulating film makes it very sensitive. In 

 fact a single cell of a battery cannot be employed as a de- 

 tector, because it is of too high a voltage for the film to 

 stand. A fraction of a volt is employed, by a potentiometer 

 device — usually something like one-tenth of a volt — and it 

 is adjusted to suit 1 in umstances. The battery acts through 

 the coherer direct on a low resistance recorder, and the 

 record on the strip shows every character of the arriving 

 pulses, and exhibits any defect in the signalling. Provided 

 that every joint and contact, except the one intended to be 

 filmed, is thoroughly good, the coherer in this form is so 

 definite and satisfai tory that it becomes safe to say that the 

 only outstanding defects are those which occur at the send- 

 ing end. The signals are picked up and recorded precisely 

 as they are emitted, as has been tested by intercalating a 

 siphon recorder in a much diluted tapping circuit at the 

 sending end, so as to get a record with which to make 

 comparison. The traces obtained at the two ends are 

 identical to a surprising degree. 



The mercury level has an adjustment which is easily 

 made. One precaution is to keep the rim of the wheel 

 clear of dust, which is done by a cork or leather pad pressed 

 lightly against it by a spring. 



The instrument is not at all sensitive to tremor, and re- 

 quires no particular delicacy of adjustment. The wheel has 

 to be positive, the mercury negative. 



A telephone in circuit, through a transformer or other- 

 wise, affords an easy method of occasionally discriminating 

 the signals by ear. The speed of the wheel gives another 

 convenient adjustment to suit various circumstances. 



A simple laboratory form of the instrument, driven by a 

 thread from Morse clockwork, can be placed in circuit with 

 a simple form of potentiometer and a siphon recorder, and 

 used for Hertz-wave investigation purposes. It is connected 

 w.th the collecting areas through a transformer, the coils 

 of the recorder being in that case shunted by means of a 

 condenser, so as to allow the full effect of the impulse to 

 be felt at the film without having to overcome anything of 

 the nature of a choke coil or other obstruction, in cases where 

 sensitiveness is desirable. 



NO. 1747, VOL. 67] 



Royal Astronomical Society, April 8. — Piof. H. H. 

 Turner, president, in the chair. — Prof. Sampson gave an 

 account of the Almucantar erected under his supervision at 

 the Durham Observatory, and described the instrumental 

 errors, and methods of adjustment of the instrument, and 

 the observations made with it during 1902. — The Astro- 

 nomer Royal exhibited photographs of the recent sun- 

 spots, and curves showing the terrestrial magnetic disturb- 

 ances which had accompanied the outbreak of solar activity. 

 In the course of the discussion Prof. Turner showed a photo- 

 graph of solar faculae, &c, taken by Prof. G. E. Hale with 

 the spectroheliograph at the Yerkes Observatory. — Mr. 

 I". A. Bellamy read a paper on the new star in Gemini 

 found by Prof. Turner from an examination of astro- 

 graphic plates taken at the Oxford University Observatory. 

 There was no trace of the star on plates taken February 

 21 and 2S, but on March 16 it appeared as of the seventh 

 magnitude. Prof. Pickering had since examined the plates 

 taken at Harvard Observatory, and found an image of the 

 star on a photograph of March 6, though there was no 

 trace (if it on earlier plates. On March 6 the Nova was 

 of the fifth magnitude ■ it had therefore considerably 

 diminished in brightness when found at Oxford, and 

 appeared to be still slowly becoming fainter. The spectrum 

 showed many bright lines. — Father Goetz gave an account 

 of observations proposed to be made at a new observatory 

 to be established in Bulawayo, Rhodesia, and of which he 

 was about to take charge. — Prof. Michie Smith described 

 the new observatory at Kodaikanal, in southern India, 

 illustrated by photographs of the observatory and its 

 surroundings, and gave a brief account of the observations 

 being made there. 



Entomological Society, April 1. — Prof. E. B. Poulton, 

 F.R.S., president, in the chair. — Mr. M. Jacoby exhibited 

 specimens of Rhagiosoma madagascariensis, Heyd., from 

 Madagascar, and Carpophagus Banksiae, McLeay, and Mecy- 

 nodera coxalgica, Boisd., from Australia. In appearance they 

 presented many charai teristics not usually associated with 

 Phytophagous Coleoptera. — Mr. C. P. Pickett exhibited 

 specimens of Dilina tiliae bred from Essex pupx', showing 

 the effects of forcing. — Mr. W. J. Lucas exhibited lantern 



1I1 of the specimen of Hemianax ephippiger, and of the 

 Orthetium species attacked by an Asilid fly, shown by Mr. 

 R. M. I. uhlan at the last meeting. — Dr. T. A. Chapman 

 re.nl contributions to the life-history of Orina (Chrysochloa) 

 tristis, var. smaragdina. — Mr. !•'. Enock read a paper, 

 illustrated with lantern slides, on the life-history of 

 Cicindela campestris. — Sir George Hampson read a paper 

 on Apoprogonia hespcrioides, a remarkable new lepidop- 

 terous insect from Zululand. He said that tin- genus must 

 be referred to the Euschemonidte, which is represented by 

 the single species Euschemon rafflesiae, Westw., from 

 Australia. In what quarter of the globe the family origin- 

 ated it was impossible to say, but the appearance oi the 

 species in question suggested that it was a survival of the 

 scattered remnant of the Antarctic fauna. It was, however, 

 most remarkable that the genus should occur in Africa and 

 Australia alone. 



Royal Meteorological Society, April 15. — Captain D. 

 Wilson-Barker, president, in the chair. — Mr. F. J. Brodie 

 read a paper on the prevalence of gales on the coasts of the 

 British Islands during the thirty years 1871-1900, being a 

 continuation of a paper on the same subject which he com- 

 municated to the Society last year. The total number of 

 gales dealt with during this period was 1455, the yearly- 

 average being 48-5, of which 106 were severe. The present 

 paper deals with (1) the number of gales experienced on the 

 west, north, south, and east coasts respectively, (2) the 

 prevalence of gales at different times in the year, and (3) 

 the mean direction from which gales blow on various parts- 

 of our coasts. — A paper on the duration of rainfall, by Mr. 

 J. Baxendell, was read by the secretary. In this paper 

 the author refers to various patterns of self-recording rain- 

 gauges, and points out the defects inherent to them, and 

 also states that it is hardly possible to determine from them 

 the rate at which rain falls, especially in very small quanti- 

 ties. From a Halliwell's self-recording rain-gauge which 

 had been in operation at Southport during 1902, the totaf 

 duration of rainfall for the year was 640 1 hours. The 

 author showed that the hourly duration values give a 



