induction coil was sent through them.'[1] These observations did not attract much attention until Professor E. Branly, of Paris, in 1890 and 1891, repeated them on an extended scale and with great variations, making the important observation that an electric spark at a distance had a similar effect in increasing the conductivity of metallic powders.[2] Branly, however, noticed that in some cases of conductors in powder, such as the peroxide of lead or antimony, the effect of the spark was to cause a decrease of conductivity.
To Professor E. Branly unquestionably belongs the honor of giving to science a new weapon in the shape of a tube containing metallic filings or powder rather loosely packed between metal plugs, and of showing that when the pressure on the powder was adjusted such a tube may be a conductor of very high resistance, but that the electrical conductivity is enormously increased if an electric spark is made in its neighborhood. He also proved that the same effect occurred in the case of two slightly oxidized steel or copper wires laid across one another with light pressure, and that this loose or imperfect contact was extraordinarily sensitive to an electric spark, dropping in resistance from thousands of ohms to a few ohms when a spark was made many yards away.
It is curious to notice how long some important researches take to become generally known. Branly 's work did not attract much attention in England until 1892, when Dr. Dawson Turner described his own repetition of Branly's experiments with the metallic filings tube, at a meeting of the British Association in Edinburgh. In the discussion which followed. Professor George Forbes made an important remark. He asked whether it was possible that the decrease in resistance could be brought about by Hertz waves.[3]
This question shows that even in 1893 the idea that the effect of the spark on the Branly tube was really due to Hertzian waves was only just beginning to arise. The following year, however, Mr. W. B. Croft repeated Branly's experiment with copper filings before the Physical Society of London, and entitled his short paper 'Electric Radiation on Copper Filings.'[4] He exhibited a tube containing copper filings loosely held between two copper plugs and joined in series with a galvanometer and cell. The effect of an electric spark at a distance, in causing increase of conductivity, was shown, and the return of the tube to its non-conducting state when tapped was also noticed.
- ↑ See also Journal de Physique, Vol. V., p. 573, 1886.
- ↑ See Comptes Rendus, Vol. CXI., p. 785; Vol. CXII., p. 112, 1891; or La Lumière Electrique, Vol. XL., pp. 301, 506, 1891; or The Electrician, Vol. XXVII., 1891, pp. 221, 448.
- ↑ See The Electrician, Vol. XXIX., 1892, pp. 397 and 432.
- ↑ Mr. W. B. Croft, Proc. Phys. Soc, Vol. XII., p. 421. Report of meeting on October 27, 1893.
