French physicist and inventor (nineteenth century)
Branly, EDOUARD, a French physicist and inventor of the coherer employed in wireless telegraphy, b. at Amiens, October 23, 1846. After receiving his early education at the Lycee of St.-Quentin, his scientific studies were begun at the Lycee Henri IV at Paris, and in 1865 he entered the Ecole Normale Superieure. In 1868 he became Licentiate in mathematics and physical science, and also agrege in physical and natural science. After occupying a professor's chair at the Lyee of Bourges, he was appointed chef des travaux in 1869 and four years later he was made director of the Laboratory of Instruction in the department of physics at the Sorbonne. In the same year (1873) he won the doctorate in science with a thesis entitled "Electrostatic Phenomena in Voltaic Cells". In 1876 he resigned his post at the Sorbonne to become professor of physics at the Catholic University in Paris. He then took up the study of medicine, obtaining his degree in 1882, and thereafter divided his time between the practice of medicine, especially of physiotherapy and electrotherapy, and his researches in physics at the Catholic University.
Dr. Branly is best known by his researches concerning radio-conductors, and particularly by his so-called coherer. He began his studies in this field in 1890, being led to undertake them by observing the anomalous change in the resistance of thin metallic films when exposed to electric sparks. Platinum deposited upon glass was first employed. The effect was at first attributed to the influence of the ultra violet light of the spark. The variations in the resistance of metals in a finely divided state were even more striking, and they were shown by Dr. Branly to be due to the action of the electrical, or Hertzian, waves of which the spark was the source. The phenomenon was investigated at great length, and further experiment led to the coherer, which is simply a glass or ebonite tube containing metallic filings which connect the two ends of a wire conductor entering the tube. When the tube is made part of a battery circuit, the filings ordinarily offer a very great resistance to the passage of a current. But if a spark be produced in the neighborhood between the terminals of an induction coil, or by the discharge of a Leyden Jar, the resistance of the filings is diminished, being no longer measured in millions but in hundreds of ohms. Upon tapping the tube the filings regain their normal resistance. This simple device was employed by Lodge in his researches and formed an important part of Marconi's successful system of wireless telegraphy. In fact the coherer first made wireless telegraphy possible. It serves as a receiver, being placed in series with a relay actuating a Morse sounder.
When electrical waves, sent out at a distant station according to an established code, impinge upon it, its resistance diminishes sufficiently to enable the relay to act and this in turn reproduces the signals in the sounder. A tapper automatically restores the resistance of the filings. Dr. Branly has given the name of radio-conductors to bodies which, like filings, can be made conductors or non-conductors at will. A number of other forms have since been devised, and he himself has found that the tripod coherer, composed of a metal disk making contact with a polished steel plate by means of three steel legs, is more sensitive and uniform in its action than the tabe coherer. He has also applied his radio-conductors to "telemechanics without wires", i.e. to the production of divers mechanical effects at a distance by means of electrical waves. Among Dr. Branly's other researches have been those relating to the effect of ultra violet light upon positively and negatively charged bodies (1890-93), electrical conductivity of gases (1894), etc. It may be noted that the germ of the "antenna:", employed particularly in long distance telegraphy, may be found in his papers published in 1891.
Dr. Branly became Commander of the Order of St. Gregory the Great in 1899 and was nominated Chevalier of the Legion of Honor in 1900 for "having discovered the principle of wireless telegraphy". He received the grand prix at the Paris Exposition, 1900, for his radio-conductors, and the prix Osiris, in 1903, from the Syndicate of the Press. He was also made a titular member of the Pontifical Academy dei Nuovi Lincei. Besides his papers published chiefly in the "Comptes Rendus", Dr. Branly is the author of a "Cours elementaire de physique" (5th ed., 1905); and "Traite elementaire de physique" (3d ed., 1906). For various types of coherer and other apparatus employed in wireless telegraphy, cf. Collins, "Wireless Telegraphy" (New York, 1905).
H. M. BROCK