Page:Popular Science Monthly Volume 63.djvu/557

Rh 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, antimony and of arsenic, and a few other metals, have a similar property. Branly tubes, therefore, made with these materials, or any arrangements 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 Schäfer. Schäfer'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 transverse 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 experiment, and they convey electricity across from side to side. Hence a current passes, having a magnitude of a few milliamperes. If, however, 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. Of recent times, the subject came back into notice through the researches of Professor E. Rutherford, who carried out at Cambridge, England, in 1896, a