Page:Collected Physical Papers.djvu/34

14 The sensitiveness of the Receiver depends greatly on the proper adjustment of the E. M. F. For all-round work, an E. M. F. of about ·45 volt, is best suited with the particular coherer used in my experiments. This was obtained from an Iron-Zinc cell, the Iron being immersed in a solution of ferric chloride, and Zinc in dilute sulphuric acid. After the Receiver has been subjected to radiation for a length of time, its sensibility is diminished; this may be restored by slightly increasing the E. M. F. The Receiver is placed in a derived circuit, the main current, and therefore the derived E. M. F., being varied by means of a rheostat.

Method of experiment

When the spark-gap is vertical, the electric radiation is to a great extent polarised, the vibration being accompanied by electric force in a vertical plane, and magnetic force in a horizontal plane. I shall, for simplicity, consider only the electric vibration. When the partially-polarised radiation is transmitted through a horizontal grating, the emergent beam is completely polarised, the vibration taking place in a vertical plane passing through the axis.

The spiral-spiring coherer itself exhibits selective absorption. It absorbs more readily vibrations parallel to its length. Thus, when the spark-gap and the coherer are parallel, the response is very energetic, whereas the response is but feeble when the two are crossed. The analysing action of the coherer becomes more complete if it be further provided with an Analyser grating with wires perpendicular to the length of the coherer.