444 POPULAR SCIENCE MONTHLY. 



induction coil was sent through them.'* 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 varia- 

 tions, making the important observation that an electric spark at a 

 distance had a similar effect in increasing the conductivity of metallic 

 powders. t 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 imquestionably 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 con- 

 ductivity 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 ex- 

 traordinarily 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 atten- 

 tion 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 dis- 

 cussion which followed. Professor George Forbes made an important 

 remark. He asked whether it was possible that the decrease in resist- 

 ance could be brought about by Hertz waves. J 



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. '§ He exhibited a tube containing copper filings loosely 

 held between two copper plugs and joined in series with a galva- 

 nometer 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 dc Physique, Vol. V., p. 573, 1886. 



t See Compfes Rendm, Vol. CXI., p. 785; Vol. CXII., p. 112, 1891; or 

 La Lumiere Electrique, Vol. XL., pp. 301, 506, 1891; or The Electrician, Vol. 

 XXVII., 1891, pp. 221, 448. 



I See The Electrician, Vol. XXIX., 1892, pp. 397 and 432. 



§Mr. W. B. Croft, Proc. Phijs. Soc, Vol. XII., p. 421. Report of meeting 

 on October 27, 1893. 



