1897.] on the Polarisation of the Electric Bay. 295 



to so great an extent by a change in tlie nature of the surface. But even 

 here the disintegration of the sparking surface produced by a torrent 

 of sparks soon puts an end to oscillation. I found this difficulty 

 removed to a great extent by making the balls of platinum, which 

 resists the disintegrating action. I also found that it was not at all 

 necessary to have a series of useless sparks, which ultimately spoils the 

 efficiency of the radiator and makes its action uncertain. A flash 

 of radiation for an experiment is obtained from a single spark, and 

 for a series of experiments one does not require more than fifty or a 

 hundred sparks, which do not in any way affect the radiator. As 

 an electric generator I use a small and modified form of Ruhmkorff's 

 coil, actuated by a single storage cell. A spark is produced by a short 

 contact and subsequent break of a tapping key. With these modi- 

 fications one of the most troublesome sources of uncertainty is 

 removed. The coil and the cell are inclosed in a small double- 

 walled metallic box, with a tube for the passage of the electric beam. 

 The magnetic variation due to the make and break of the primary 

 of the Ruhmkorff's coil, disturbs the receiver. This difficulty is 

 removed by making the inner box of soft iron, which acts as a 

 magnetic screen. 



A few words may here be said about the necessary conditions to 

 be kept in view in making an electric wave apparatus an instrument 

 of precision. If one merely wishes to produce response in a receiver 

 at a distance, the more energetic the vibration is, the more likely 

 it is to overcome obstacles. The waves may with advantage be of 

 large size, as they possess very great penetrative power. The 

 surface or the depth of the sensitive layer in the receiver may be 

 extended, for if one part of it does not respond another part will. But 

 for experimental investigations the conditions to be fulfilled are quite 

 different. Too great an intensity of radiation makes it almost impos- 

 sible to prevent the disturbance due to stray radiation. As the waves 

 are invisible, it is difficult to know through what unguarded points they 

 are escaping. They may be reflected from the walls of the room or 

 the person of the experimenter, and falling on the receiver disturb it. 



The radiation falling on any portion of the receiving circuit — the 

 leading wires or the galvanometer — disturbs the delicate receiver. 

 It is extremely difficult to shield the receiving circuit from the dis- 

 turbing action of stray radiation. These difficulties were, however, 

 successfully removed by the use of short electric waves. With these, 

 it is not at all necessary to take special precautions to shield either 

 the galvanometer or the leading wires, the sensitive layer in the 

 receiver alone being affected by the radiation. The bare leading 

 wires may be exposed in close proximity to the source of radiation, 

 and yet no disturbance is produced. 



For experimental investigations it is also necessary to have a narrow 

 pencil of electric radiation, and this is very difficult to obtain, unless 

 waves of very short length are used. With large waves diverging 

 in all directions and curling round corners, all attempt at accurate 

 work is futile. For angular measurements it is necessary to direct 



X 2 



