Page:On a Complete Apparatus for the Study of the Properties of Electric Waves.djvu/5

Rh key. The radiating apparatus may thus be made very small and portable, and requires very little attention. After the storage cell is once charged, experiments may be carried on for days, a flash of radiation being produced at any time by merely manipulating the key.

For a detector of radiation I used a form of Prof. Lodge's coherer. At first I used one made of metallic filings, originally discovered by M. Branly; but great difficulty was experienced in making the receiver respond to different vibrations, and in the capriciousness of its response. The difficulty was still further enhanced when the radiator and the receiver had to be enclosed in narrow tubes to enable angular measurements to be made with any accuracy. It seemed to me that the frequent loss of sensibility might be due to the particles getting jammed together, and the fatigued condition of contact surfaces. In order to avoid this I used a layer of narrow spirals of steel, lying side by side, and rolling on a smooth surface. The points of contact are numerous, and fresh surfaces can be brought into action by a slight rolling of the spirals. By this spiral-spring arrangement the pressure exerted on contiguous spirals is also made fairly uniform.

From a series of experiments carried out to determine the other causes which may be instrumental in producing loss of sensibility, I found that the sensibility of the receiver to a given radiation depends (1) on the pressure to which the spirals are subjected, and (2) on the E.M.F. acting in the circuit. The pressure on the spirals may be adjusted, as will be described later on, by means of a fine screw. The E.M.F. is varied by