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 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 in. sphere with sparks to ends of a diameter ([Fig. 19]), an arrangement which emits 7 in. 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 in. hollow cylinder with spark knobs at ends of an internal diameter ([Fig. 12]). This last emits 3 in. 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 crossways, which makes it less sensitive, and has also the advantage of doing away with the polarising, or rather analysing, power of a crossway tube.
The radiation from the sphere is still too strong, but it can be stopped down by a diaphragm plate with holes in it of varying size clamped on the sending box ([right-hand side of Fig. 21]).
Reflection.
Having thus reduced the excursion of the spot of light to a foot or so, a metal plate is held as reflector, and at once the spot travels a couple of yards. A wet cloth reflects something, but a thin glass plate, if dry, reflects next to nothing, being, as is well known, too thin to give anything but “the black spot.” I have fancied that it reflects something of the 3 in. waves.
With reference to the reflecting power of different substances, it may be interesting to give the following numbers showing the motion of the spot of light when 8 in. waves were reflected into the copper hat, the angle of incidence being about 45 deg., by the following mirrors:—
| Sheet of window glass | 0 | or at most | 1 division. |
| Human body | 7 | divisions. | |
| Drawing board | 12 | ” | |
| Towel soaked with tap-water | 12 | ” | |
| Tea-paper (lead?) | 40 | ” | |
| Dutch metal paper | 70 | ” | |
| Tinfoil | 80 | ” | |
| Sheet copper | 100 | and up against stops. | |