A heliograph is a wireless telegraph that signals by flashes of sunlight reflected by a mirror. The flashes are produced by momentarily pivoting the mirror, or by interrupting the beam with a shutter; the heliograph was a simple but effective instrument for instantaneous optical communication over long distances during the late 19th and early 20th century. Its main uses were military and forest protection work. Heliographs were standard issue in the British and Australian armies until the 1960s, were used by the Pakistani army as late as 1975. There were many heliograph types. Most heliographs were variants of the British Army Mance Mark V version, it used a mirror with a small unsilvered spot in the centre. The sender aligned the heliograph to the target by looking at the reflected target in the mirror and moving their head until the target was hidden by the unsilvered spot. Keeping their head still, they adjusted the aiming rod so its cross wires bisected the target, they turned up the sighting vane, which covered the cross wires with a diagram of a cross, aligned the mirror with the tangent and elevation screws so the small shadow, the reflection of the unsilvered spot hole was on the cross target.

This indicated. The flashes were produced by a keying mechanism that tilted the mirror up a few degrees at the push of a lever at the back of the instrument. If the sun was in front of the sender, its rays were reflected directly from this mirror to the receiving station. If the sun was behind the sender, the sighting rod was replaced by a second mirror, to capture the sunlight from the main mirror and reflect it to the receiving station; the U. S. Signal Corps heliograph mirror did not tilt; this type produced flashes by a shutter mounted on a second tripod. The heliograph had some great advantages, it allowed long distance communication without a fixed infrastructure, though it could be linked to make a fixed network extending for hundreds of miles, as in the fort-to-fort network used for the Geronimo campaign. It was portable, did not require any power source, was secure since it was invisible to those not near the axis of operation, the beam was narrow, spreading only 50 feet per mile of range.

However, anyone in the beam with the correct knowledge could intercept signals without being detected. In the Boer War, where both sides used heliographs, tubes were sometimes used to decrease the dispersion of the beam. In some other circumstances, though, a narrow beam made it difficult to stay aligned with a moving target, as when communicating from shore to a moving ship, so the British issued a dispersing lens to broaden the heliograph beam from its natural diameter of 0.5 degrees to 15 degrees. The range of a heliograph depends on the opacity of the air and the effective collecting area of the mirrors. Heliograph mirrors ranged from 1.5 inches to 12 inches or more. Stations at higher altitudes benefit from thinner, clearer air, are required in any event for great ranges, to clear the curvature of the earth. A good approximation for ranges of 20–50 miles is that the flash of a circular mirror is visible to the naked eye for 10 miles for each inch of mirror diameter, farther with a telescope.

The world record distance was established by a detachment of U. S. signal sergeants by the inter-operation of stations on Mount Ellen and Mount Uncompahgre, Colorado, 183 miles apart on September 17, 1894, with Signal Corps heliographs carrying mirrors only 8 inches square. The German professor Carl Friedrich Gauss of the University of Göttingen developed and used a predecessor of the heliograph in 1821, his device directed a controlled beam of sunlight to a distant station to be used as a marker for geodetic survey work, was suggested as a means of telegraphic communications. This is the first reliably documented heliographic device, despite much speculation about possible ancient incidents of sun-flash signalling, the documented existence of other forms of ancient optical telegraphy. For example, one author in 1919 chose to "hazard the theory" that the mainland signals Roman emperor Tiberius watched for from Capri were mirror flashes, but admitted "there are no references in ancient writings to the use of signaling by mirrors", that the documented means of ancient long-range visual telecommunications was by beacon fires and beacon smoke, not mirrors.

The story that a shield was used as a heliograph at the Battle of Marathon is a modern myth, originating in the 1800s. Herodotus never mentioned any flash. What Herodotus did write was that someone was accused of having arranged to "hold up a shield as a signal". Suspicion grew in the 1900s; the conclusion after testing the theory was "Nobody flashed a shield at the Battle of Marathon". In a letter dated 3 June 1778, John Norris, High Sheriff of Buckinghamshire, notes: "Did this day heliograph intelligence from Dr Franklin in Paris to Wycombe". However, there is little evidence that "heliograph" here is other than a misspelling of "holograph"; the term "heliograph" for solar telegraphy did not enter the English language until the 1870s—even the word "telegraphy" was not coined until the 1790s. Henry Christopher Mance, of the British Government Persian Gulf Telegraph Department, developed the first accepted heliograph about 1869 while stationed at Karachi, in the Bombay Presidency in British India.

Mance was familiar with heliotropes by their use for the Great India Survey. The Mance Heliograph was operated by one man, since it weighed about seven pounds, the operator co

Lowell S. Brown

Lowell S. Brown is an American theoretical physicist, a retired Staff Scientist and Laboratory Fellow at Los Alamos National Laboratory, Professor Emeritus of physics at University of Washington, he was a student of Julian Schwinger at Harvard University and a recipient of the John Simon Guggenheim Memorial Foundation Fellowship. Brown authored a book on Quantum Field Theory that has received over 5,000 citations, he has authored or co-authored over 150 articles that have accumulated over 11,000 citations. Lowell S. Brown earned his A. B. in physics from the University of California, Berkeley in 1956 and his Ph. D. in physics from Harvard University in 1961, with a National Science Foundation Predoctoral Fellowship, studying quantum field theory under Julian Schwinger. After National Science Foundation Postdoctoral Fellowships at Istituto di Fisica Dell'Universita in Rome and at Imperial College of Science and Technology in London, Brown joined Yale University as an Associate Professor through 1968.

For most of his career, Brown served as a Professor at the University of Washington. He was a Staff Scientist at Los Alamos National Laboratory in New Mexico, he is now a Guest Scientist at Los Alamos. Brown again visited Imperial College in 1971-1972, continuing his research with a National Science Foundation Senior Postdoctoral Fellowship, he was awarded a John Simon Guggenheim Memorial Foundation Fellowship in 1979 and undertook his research at the European Organization for Nuclear Research, Geneva and the Institute for Advanced Study, New Jersey. Most of Brown's work has involved quantum field theory applied to elementary particle physics, general relativity, plasma physics, atomic physics, nuclear physics, his book "Quantum Field Theory" has been well received. Pierre Ramond's review in Science states that Brown's book is "marked by its astute choice of topics as well as by the clarity with which they are expounded, it is akin to a toolbox for students of modern quantum field theory... a thorough and rare treatment…a interesting and original textbook.

I recommend this book to whoever aspires to become either a particle or a condensed matter physicist."Brown's work on the interaction of intense laser beams with electrons is still cited forty or so years later. In astrophysics and general relativity, his work on the stress-energy tensor of various fields coupled to an arbitrary classical gravitational field is noteworthy: it uses the method of dimensional continuation and proper time representations, with these methods, he computed the unique gravitational anomaly for scalar fields and the anomaly for vector fields. Brown and collaborators computed the energy-energy correlation in electron-positron annihilation, which provides one method of measuring the strong interaction QCD coupling constant. Brown was the first to compute the stress-energy tensor between conducting planes; the stress tensor evaluated on a plane yields the Casimir force. He was the first to exhibit the classical limit of the hydrogen atom, he constructed large-quantum-number wave packets that spread while moving in circular orbits.

At the University of Washington, Hans Dehmelt captured single charged particles in stable orbits in a Penning trap. This arrangement, called geonium, enabled measurement of the magnetic moment of the electron with exquisite precision for which Dehmelt won the Nobel Prize. Brown became fascinated with this new experimental procedure and with coworkers wrote many papers investigating the detailed workings of geonium, his work culminated in a long review article that has become a handbook for other experimenters who use a Penning trap. Brown investigated plasma effects on nuclear fusion, wrote a paper applying field theory to plasma physics, obtained the non-leading corrections in plasma stopping power, provided an effective field description for deuterium-tritium fusion. Brown, Lowell S.. Quantum Field Theory. Cambridge University Press. ISBN 978-0-521-46946-3. Brown, L. S.. "Interaction of intense laser beams with electrons". Physical Review. 133: a705. Bibcode:1964PhRv..133..705B. Doi:10.1103/physrev.133.a705.

Brown, L. S.. "Stress-tensor trace anomaly in a gravitational metric: Scalar fields". Physical Review D. 15: 1469–1483. Bibcode:1977PhRvD..15.1469B. Doi:10.1103/physrevd.15.1469. Brown, L. S.. "Stress-tensor trace anomaly in a gravitational metric: General theory, Maxwell field". Physical Review D. 15: 2810. Bibcode:1977PhRvD..15.2810B. Doi:10.1103/physrevd.15.2810. Basham, C. L.. T.. "Energy Correlations in Electron-Positron Annihilation: Testing Quantum Chromodynamics". Physical Review Letters. 41: 1585. Bibcode:1978PhRvL..41.1585B. Doi:10.1103/physrevlett.41.1585. Basham, C. L.. T.. "Energy correlations in electron-positron annihilation in quantum chromodynamics: Asymptotically free perturbation theory". Physical Review D. 19: 2018. Bibcode:1979PhRvD..19.2018B. Doi:10.1103/physrevd.19.2018. Brown, L. S.. "Vacuum stress between conducting plates: an image solution". Physical Review. 184: 1272. Bibcode:1969PhRv..184.1272B. Doi:10.1103/physrev.184.1272. Brown, L. S.. "Classical limit of the hydrogen atom". American Journal of Physics.

41: 525–530. Bibcode:1973AmJPh..41..525B. Doi:10.1119/1.1987282. Brown, L. S.. "Geonium theory: Physics of a single electron or ion in a Penning trap". Reviews of

List of chancellors and vice-chancellors of the University of Pretoria

These are the former and present vice-chancellors and chancellors of the University of Pretoria, South Africa. The vice-chancellor and principal is the head of the university, supported by four deputy vice-chancellors. Former and current persons who have fulfilled the position are: A C Paterson: 1918–1924 N M Hoogenhout:1925–1927 A E du Toit:1927–1934 C F Schmidt:1935–1940 M C Botha:1941–1947 C H Rautenbach:1948–1970 E M Hamman:1970–1981 D M Joubert: 1982–1991 Flip P Smit: 1992–1996 J van Zyl: 1997–2001 Calie Pistorius: 2001–2009 Cheryl de la Rey: 2009 Tawane Kupe: 2019 The Chancellor is the titular head of the university. Former and current persons who have fulfilled the position are: Justice Tielman Roos, Supreme Court of Appeal and South African Minister of Justice: 1930 – 1932 Rev Adriaan Louw: 1933 – 1934 Dr Hendrik van der Bijl and industrialist, founding chairman of Eskom and founder of ISCOR: 1934 – 1948 Advocate Charles Theodore Te Water, South African diplomat and President of the League of Nations: 1949 – 1964 Dr Hilgard Muller, Mayor of Pretoria and Minister for Foreign Affairs: 1965 – 1984 The Honourable Alwyn Schlebusch, Minister of Public Works and Immigration, Minister of Justice and Internal Affairs, Minister in the Office of the President and South Africa's only Vice State President: 1984 – 1986 Dr Anton Rupert, South African entrepreneur and billionaire: 1987 – 1992 Dr Chris Stals, Governor of the South African Reserve Bank and multiple company directorships: 1992 – 2005 Prof Wiseman Nkuhlu, South Africa's first black chartered accountant, economic advisor to the President, Chairman for the NEPAD steering committee, Chairman of the Development Bank of Southern Africa and multiple company directorships: 2006 to present List of South African university chancellors and vice-chancellors