Signalling through space without wires : being a description of the work of Hertz & his successors . out-side ordinary galvanometer, it remains nearly as sensitive as * FitzGerald tells me that he has succeeded with carbon also. Myexperience is that the less oxidisable the metal, the more sensitive and alsothe more troublesome is the detector. Mr. Robinson has now made me ahydrogen vacuum tube of brass filings, which beats the coherer foisensitiveness. July, 1894. WORK OF HERTZ LECTURE. 35 it was before (nearly, not quite), for the circuit picks up thewaves and they run along the insulated wir
Signalling through space without wires : being a description of the work of Hertz & his successors . out-side ordinary galvanometer, it remains nearly as sensitive as * FitzGerald tells me that he has succeeded with carbon also. Myexperience is that the less oxidisable the metal, the more sensitive and alsothe more troublesome is the detector. Mr. Robinson has now made me ahydrogen vacuum tube of brass filings, which beats the coherer foisensitiveness. July, 1894. WORK OF HERTZ LECTURE. 35 it was before (nearly, not quite), for the circuit picks up thewaves and they run along the insulated wires into the closedbox. To screen it effectively, it is necessary to enclose batteryand galvanometer and every bit of wire connection ; the onlything that may be left outside is the needle of the galvano-meter. Accordingly, here we have a compact arrangement ofbattery and galvanometer coil and coherer, all shut up in acopper box (Fig. 19c). The galvanometer coil is fixed againstthe side of the box at such height that it can act convenientlyon an outside suspended compass needle. The slow magnetic. Fia. 19c.—Protected Detector. A is an occasional wire passing throughshuttered aperture. E is a lead tube enclosing leading wires, as in Fig. 21. action of the current in the coil has no difficulty in gettingthrough copper, as everyone knows : only a perfect conductorcould screen off that; but the Hertz waves are effectively keptout by the sheet copper. It must be said, however, that the box must be exceedinglywell closed for the screening to be perfect. The very narrowestchink permits their entrance, and at one time I thought I shouldhave to solder a lid on before they could be kept entirely a copper lid on to a flange in six places was notenough. But by the use of pads of tinfoil and tight clamping, d 2 36 SIGNALLING WITHOUT WIRES. chinks can be avoided, and the inside of the box becomes thenelectrically dark. If even an inch of the circuit protrudes, it at once become
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