3U 



Professor Oliver Lodge 



[June 1, 



suspended compass needle. The slow action of the coil has no 

 difficulty in getting through copper, as every one knows ; only a 

 perfect conductor could screen otf that, but the Hertz waves are 

 effectively kept out by sheet copper. 



It must be said, liowever, that the box must be exceedingly well 

 closed for the screening to be perfect. The very narrowest chink 

 permits their entrance, and at one time I thought I should have to 

 solder a lid on before they could be kept out entirely. Clamping a 

 copper lid on to a flange in six places was not enough. But by the 

 use of pads of tinfoil, chinks can be avoided, and the inside of the box 

 becomes then electrically dark. 



If even an inch of the circuit protrudes, it at once becomes 



Fig. 20. 



Spherical Radiator for emitting a Horizontal Beam, arranged inside a 

 Copper Hat, fixed against the outside of a metal-lined box. One- 

 eighth natural size. The wires pass to the coil and battery inside 

 the box through glass tubes not shown. 



slightly sensitive again; and if a mere single wire protrudes through 

 the box, provided it is insulated where it passes through, the waves 

 will utilise it as a speaking tube, and run blithely in. And this 

 whether the wire be connected to anything inside or not, though it 

 acts more strongly when connected. 



In careful experiments, where the galvanometer is protected in 

 one copper box and the coherer in another, the wires connecting the 

 two must be encased in a metal tube, Fig. 21, and this tube must be 

 well connected with the metal of both enclosures, if nothing is to get 

 in but what is wanted. 



Similarly, when definite radiation is desired, it is well to put the 

 radiator in a copper hat, open in only one direction (Fig. 20). And in 



