1894.] on the Work of Hertz. 345 



order to guard against reflected and collateral surgings running along 

 the wires which pass outside to the exciting coil and battery, as they are 

 liable to do, I am accustomed to put all these things in a packing case 

 lined with tinfoil, to the outside of which the sending hat is fixed, 

 and to pull the key of the primary exciting circuit by a string from 

 outside. 



Even then, with the lid of the hat well clamped on, something 

 gets out, but it is not enough to cause serious disturbance of quali- 

 tative results. The sender must evidently be thought of as emitting 

 a momentary blaze of light which escapes through every chink. Or, 

 indeed, since the waves are some inches long, the difficulty of keeping 

 them out of an enclosure may be likened to the difficulty of excluding 

 sound ; though the difficulty is not quite so great as that, since a 

 reasonable thickness of metal is really opaque. I fancied once or 

 twice I detected a trace of transparency in such metal sheets as 

 ordinary tinplate, but unnoticed chinks elsewhere may have deceived 

 me. [Further investigation fails to detect real transparency even in 

 good tinfoil.] 



One thing in this connection is noticeable, and that is how little 

 radiation gets either in or out of a small round hole. A narrow long 

 chink in the receiver box lets in a lot • a round hole the size of a 

 shilling lets in hardly any, unless indeed a bit of insulated wire 

 protrudes through it like a collecting ear trumpet. 



It may be asked how the waves get out of the metal tube of an 

 electric gas-lighter. But they do not ; they get out through the 

 handle, which being of ebonite is transparent. Wrap up the handle 

 tightly in tinfoil, and a gas-lighter is powerless. 



Optical Experiments. 



And now in conclusion I will show some of the ordinary optical 

 experiments with Hertz waves, using as source either one of two 

 devices ; either a 5-inch sphere with sparks to ends of a diameter, 

 Fig. 19, an arrangement which emits 7-inch waves but of so dead-beat 

 a character that it is wise to enclose it in a copper hat to prolong 

 them and send them out in the desired direction ; or else a 2-incJi 

 hollow cylinder with spark knobs at ends of an internal diameter, 

 Fig. 12. This last emits 3-inch waves of a very fairly persistent 

 character, but with nothing like the intensity of one of the outside 

 radiators. 



As receiver there is no need to use anything sensitive, so I 

 employ a glass tube full of coarse iron filings, put at the back of 

 a copper hat with its mouth turned well askew to the source, which 

 is put outside the door at a distance of some yards, so that only a 

 little direct radiation can reach the tube. Sometimes the tube is put 

 lengthways in the hat instead of cross ways, which makes it less sen- 

 sitive, and has also the advantage of doing away with the polarising, 

 or rather analysing, power of a crossway tube. 



