Wireless Waves on a Shielded Frame Aerial. 505 



tlie side of the box was completely open ; but with the box 

 so placed that its open side pointed in a direction making 

 an angle of 45 degrees with the direction of Paris, with 

 the frame pointing in the same direction or more towards 

 Paris, the signals could be heard, though faintly, being- 

 reduced to some 1 per cent, of their value with the frame 

 completely unshielded. 



For any signals to be heard, it was, however, essential that 

 the relative positions of the box and frame were such that a 

 prolongation of the plane of the frame towards or away from 

 Paris, no matter which, came out of the open side of the box, 

 clear of the copper sides. 



With the box turned up so that its open side pointed to the 

 sky, with the vertical frame pointing to Paris, the signals 

 could be heard at about 1 per cent, of their full unshielded 

 value, and disappeared when the lid was put on, in exactly 

 the same way as when the open side of the box was pointing 

 to, or directly away from Paris. 



From these experiments it would appear that waves of 

 the length and strength of those experimented with do not 

 penetrate into a completely closed metallic box to a sufficient 

 extent to give audible signals with a frame aerial and 

 amplifier of the description used, even if the side of the box 

 lacing the source of the waves is only closed with tin-foil. 

 Further, it would seem that the screening effect is largely 

 a matter of a closed electrical conducting circuit in that 

 portion of the box that surrounds the periphery of the frame, 

 the smallest break in the electrical conducting continuity 

 interfering largely with the screening effect. 



That this is so, is shown by further experiments made with 

 a flat sheet of tin plate, 5 feet in length and 8 inches wide, 

 bent so as to form a cylinder. This was placed over the 

 frame so that the axis of the cylinder coincided with that of 

 the frame. With the ends of the tin-plate sheet overlapping, 

 but not in electrical contact, the effect in reducing the 

 strength of the signals was small, but immediately increased 

 considerably, so that the signals were reduced to about 

 20 per cent, of their original strength, when the ends of the 

 sheet were allowed to touch so as to form a continuous 

 conducting ring round the periphery of the frame. Con- 

 tracting the diameter of the ring by sliding the overlapping 

 ends of the plate so that the ring was closer to the frame all 

 round, still further reduced the strength of the signals, till 

 when the ring was touching the insulated wires of the frame all 

 round, the signals very nearly disappeared. This indicates that 

 the effect is not merely a question of screening, but also of eddy. 



Phil Mag. S. G. Vol. 42. No. 250. Oct, 1921. 2 M 



