J. Brit. Astron. Assoc., 106, 5, 1996

Letters

(Note: The Association is not responsible for individual opinions expressed in articles, reviews, letters or reports of any kind.)


The 'Optical Lever'

From the Director of the Aurora Section

I have often been quoted in the literature as the designer of the Jamjar Magnetometer. The magnetometer depends for its accuracy on the optical lever.2 A light beam is reflected from a mirror fixed to the suspended magnet to shine on a scale at a distance, by which means small deviations in azimuth of the magnet's orientation may be detected. Recently I have been following up the history of magnetometry to determine the original inventor of the optical lever, both in the UK and in Germany, and have met so far with a degree of uncertainty among the references and the authorities with whom I have corresponded.

It would appear that in 1826 J. C. Poggendorff invented a method of reading a scale by means of a mirror, which C. F. Gauss incorporated in his standard absolute magnetometers. W. E. Weber was using this method until 1880. It is uncertain that this was an optical lever as I have defined it; in fact some authorities consider that the optical lever could not have been invented until electric lighting was available to produce a steady, sharp, collimated beam.

German authorities suggest that the optical lever as such was devised by Sir William Thomson (Lord Kelvin), and it is certain that he used the principle of the lever in constructing a sensitive mirror galvanometer for detecting the small electrical currents carrying messages through the first transatlantic telegraph cable.3 I have however found an illustration of a mirror galvanometer where the light source was shown to be an oil lamp and not an electric light,4 although later pictures show galvanometers fitted with incandescent lamps.

To summarise, it would seem that in the early days of geomagnetic measurement a form of scale using a mirror and telescope was used and in later years the principle of the optical lever, having a light beam as a giant instrument needle with no inertia, in the mechanical sense, came into use generally in instrumentation of all kinds where very sensitive readings were required.6 There is still the uncertainty over who invented the principle. Would any readers be able to shed further light on this intriguing piece of instrumental history?

My thanks are already due to Messrs Gadsden, Kerridge, Simpson, Insley, Mersich and Schreiber, who have contributed much useful information in the course of correspondence.

R. J. Livesey
Flat 1/2, East Parkside, Edinburgh EH16 5XJ


Understanding space and time

From Mr V. Mayes

C. R. Dickens, in his review of Understanding Space and Time, refers to 'the twin paradox, whereby a person leaving Earth in a very fast space ship and then returning later has aged less than the twin who remained on Earth'.

I am no physicist, but I always understood that the whole point of relativity is that the point of view of all observers is equally valid, and that the laws of physics are the same for all observers. Therefore both the following statements must be true:

The point of view of Twin B is just a valid as the point of view of Twin A. If Twin A's clock runs slow because he is travelling away from Twin B with a high velocity v, then Twin B's clock must also run slow, by an exactly equal amount, because he is travelling away from Twin A with the same high velocity v. Therefore, when they meet again, the net effect of the experience must surely be the same for both? There is no universal frame of reference, so we can consider only the velocities of bodies relative to one another, in which case Twin A has no more status in the matter than Twin B.

Could any physicist reader comment?

Victor Mayes

27 Souter Court, Wardle Edge, Smallbridge, Rochdale OL12 9EZ


The exceptional fireball of 1995 July 28

From Mr Alistair McBeath

As an author of the report on the brilliant fireball of 1995 July 28 criticised by Howard Miles in his presentation to the Association's meeting of 1996 February 24, I was reminded of the comment by Berthollet, after the publication of a document by witnesses of a meteorite fall near Barbotan in France in 1791. McSween quotes this as 'How sad it is that the entire municipality enters folk tales upon an official record, presenting them as something actually seen, while they cannot be explained by physics nor by anything reasonable.'2 I had thought that the days when large numbers of independent eye-witness accounts were discarded as all being equally unreliable were past.

In choosing to discount the 'reports from the Sunderland area', Mr Miles removes the data which he commends as being the most valuable, that from witnesses directly under the projected ground track, including in this instance several dozen calls logged by the emergency services soon after the fireball passed over. Such calls were made because the fireball's detonation produced acoustic shockwaves that shook buildings, rattled objects and rumbled around the Sunderland environs, up to 30–40km away, between ~90–600 seconds later. The time delay progressively increased with distance from the most probable ground track published in a detailed preliminary analysis.3,4 This evidence is backed up by twelve reports of electrophonic sounds detected virtually simultaneously with the meteor's flight, most within 1.1km of the projected ground track. The essential correctness of the published trajectory's details was later demonstrated and confirmed by a photograph of the meteor's zero order spectrum, kindly provided by Dr Henry Soper on the Isle of Man.

No 'belief that part of the bolide came down in the Sunderland area' was ever expressed in any published analysis. As clearly stated in references 3, 4 and elsewhere, any meteorites would have fallen into the North Sea probably ~40km offshore. The severity of the acoustic effects at two locations led to searches for meteorites at Sunderland during August, before the analysis was even begun, since the fireball's severe fragmentation implied a possible chondritic composition, and chondritic meteorites are notoriously difficult to find the longer they have lain on the Earth's surface. Theoretical calculations show that the meteor's trajectory from reference 3 would have been visible above the true horizon for sites above sea level, allowing for the Earth's curvature and estimated refraction effects, from locations around 350–400km away from the brightest part of the flight, which probably occurred some ~10–20km before the end of its luminous path, but this would be heavily influenced by local sky conditions and horizon obstructions. Several reports received from near London, and one from the Netherlands, indicated a sudden bright illumination of the northern sky was detected, but no actual meteor seen.

Alistair McBeath

12A Prior's Walk, Morpeth, Northumberland NE61 2RF

Howard Miles replies:

I am surprised that Alistair McBeath did not wait until after my full report of the event had appeared in the Journal before making comment. Meeting reports are not verbatim and are, of necessity, condensed; they often produce generalisations which are basically, but not strictly, correct.

In my paper (to be published shortly) I have said that a large number of reports from the Sunderland area were collected by D. Simpson, but that he had sent me only a précis of these reports. Due to the general nature of many of them, they were of fairly limited use. Sonic reports are of great interest, but again are of limited use and often hopelessly misleading, unless backed up by accurate visual observations. I cannot comment on Mr McBeath's figures because of their general nature. I am also a little puzzled as to how Dr Henry Soper's excellent photograph of the meteor's zero order spectrum, taken from the Isle of Man, can by itself confirm or deny the accuracy of a trajectory.

As far as the second half of Mr McBeath's letter is concerned, I believe it confirms the views expressed in my paper about such reports, and the account of the event as it appeared in WGN.

Howard Miles

Lane Park, Pityme, St Minver, Wadebridge, Cornwall PL27 6PN


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