HERTZIAN \VAVE WIRELESS TELEGRAFIiY. 553 



peroxide of lead acts in an opposite manner to metallic filings, in 

 that when placed in a Branly tube it increases its resistance under the 

 action of an electric spark, instead of decreasing it. Again, Professor 

 Bose has found that fragments of metallic potassium in kerosene oil 

 behave in a similar manner, and that certain varieties of silver, anti- 

 mony and of arsenic, and a few other metals, have a similar property. 

 Branly tubes, therefore, made with these materials, or any arrange- 

 ments which act in a similar manner, have been called 'anti-coherers.' 

 The most interesting arrangement which has been called by this name 

 is that of Schafer. * Schafer's kumascope is made in the following 

 manner : A very thin film of silver is deposited upon glass and a strip 

 of this silver is scratched across with a diamond, making a fine trans- 

 verse cut or gap. If the resistance of this divided strip of silver is 

 measured, it will be found not to be infinite, but may have a resistance 

 as low as forty or fifty ohms if the strip is thirty millimeters wide. On 

 examining the cut in the strip with a microscope, it will be found that 

 the edges are ragged and that there are little particles of silver lying 

 about in the gap. If then an electromotive force of three volts or more 

 is put on the two separated parts of the strip, these little particles of 

 silver fly to and fro like the pith balls in a familiar electrical experi- 

 ment, and they convey electricity across from side to side. Hence a 

 current passes, having a magnitude of a few milliamperes. If, how- 

 ever, the strip is employed as a kumascope and connected at one end 

 to the earth and at the other end to an aerial, when electric waves fall 

 upon the aerial, the electrical oscillations thereby excited seem to have 

 the property of stopping this dance of silver particles and the resistance 

 of the gap is increased several times, but falls again when the wave 

 ceases. If therefore a telephone and battery are connected between 

 two portions of the strip, the variation of this battery current will affect 

 the telephone in accordance with the waves which fall upon the aerial, 

 and the arrangement becomes therefore a wave-detecting device. It is 

 said to have been used in wireless telegraph experiments in Germany 

 up to a distance of ninety-five kilometers. 



We must next direct attention to those wave-detecting devices which 

 depend upon magnetization of iron, and here we are able to record 

 recent and most interesting developments. More than seventy years 

 ago, Joseph Henry, in the United States, noticed the effect of an 

 electric spark at a distance upon magnetized needles.f Of recent times, 

 the subject came back into notice through the researches of Professor 

 E. Eutherford, I who carried out at Cambridge, England, in 1896, a 



* See E. Marx, Phys. Zeitschrift, Vol. II., p. 249; Science Abstracts, Vol. 

 IV., p. 471. See also German Patent Specification No. 121,663, Class 21a. 

 t See ' The Scientific Writings of Professor Joseph Henry.' 

 + Phil. Trans. Roy. Soc. Loud., 1897, Vol. 189a, p. 1. 



