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Those that light but a small area are called "concentrating shades, while the "distributing" shade spreads the light over a much larger area.

Figure 540 shows one type of shade and the change in the

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FIG. 541.-Section of Lighthouse Lenses arranged to throw Parallel Rays in Four Directions

distribution made by its use. The dotted curve gives the distribution of light from the lamp without a shade, the radial distance of the curve at any point from the center giving the candle power of the lamp in that direction. The full-line curve shows the distribution when the shade is used, the greater part of the light being thrown downward.

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568. Hertz Waves. We have already seen that a radiometer can be set in motion by invisible heat rays (§ 274), and that the sensitized film on photographic paper is affected by invisible radiations of shorter wave length than those of violet light (§ 548). We are now to consider invisible radiations set up by electrical means. If the discharge of a Leyden jar is sent through a metallic circuit and across a short air gap, it will set up a like discharge in the air gap in the circuit of a second similar jar, provided the circuits are parallel and the areas of the circuits are the same. This electrical resonance is analogous to the setting of a tuning fork in motion by the vibrations of a similar fork (§ 210). If the forks do not vibrate at the same rate, the second fork will not be put in vibration, and if the area of the second circuit is changed, no spark will appear in the second air gap. The fork is put in motion by waves of air; but electrical resonance is caused by ether waves set up by the spark in the first circuit.

We have seen that the electric spark discharge of a Leyden jar is oscillatory (§ 370). If the image of the spark is observed in a mirror revolving at high speed, it is seen to be a succession of flashes following each other at extremely short intervals.

The velocity of the ether waves set up by electrical oscillations was first determined by Heinrich Hertz in 1888, hence they are known as the Hertz waves. This velocity is the same as that of light, 300,000 km. per second. Hertz found that there were 10,000,000 oscillations per second, hence the wave length is 30 m. By reducing the size of the Leyden jar and the length of the circuit, and letting the discharge take place between two balls, the wave length can be made much shorter and the number of oscillations per second much greater.

Silver or nickel filings placed between two metal disks in a glass tube have an extremely high resistance. On the passage of an ether wave sent out by a Leyden jar spark, the filings cohere and offer little resistance. Marconi used this coherer as a detector of ether waves in wireless telegraphy, by putting it in a local circuit, containing a sounder or bell, which gave a signal on every passage of a wave from the sending station.

The sending station (Fig. 543) of a wireless telegraph system will, in general, include: a transformer, A, or a Ruhmkorff coil, the second

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B are of zinc and FIG. 543.- Diagram of a Sending Station usually inclosed in a glass globe to reduce the noise of the discharge. The secondary helix E is grounded at one end and terminates at the other end in the antenna F, a wire or

group of wires going high into the air. On closing the key K, a spark jumps across B and by properly adjusting the size of the condenser C, the turns of wire in D and E, and the distance between D and E with a given area and elevation of the antenna, the two circuits BCD and EF may be so tuned that a maximum of electrical wave energy may be radiated into space from F.

These electric waves, traveling in all directions with the speed of light, may be roughly likened to the waves set up in still water when a stone is thrown into it. Gradually lessening in amplitude, they may be perceived in any direction at a distance which depends upon the sensitiveness of the detector.

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FIG. 544.-Diagram of a
Receiving Station

The receiving station for wireless work (Fig. 544) consists of the antenna F, connected to one end of a coil H, the other end of which is grounded; a coil I, movable with respect to H; a detector J; a con

denser C, and a telephone receiver.

The feeble electric waves reaching the antenna F, many million times per second, may be heard in the telephone, upon properly adjusting H to F and I to H, as a distinct musical tone every time the key K in the sending station is closed. The very rapid oscillations of the electric waves cannot be detected by the unaided telephone receiver because the diaphragm cannot vibrate fast enough. Hence it is necessary that the detector be so constructed that the train of waves sent out each time the sending key is closed shall be received as an individual signal the length of which depends upon the length of time the sending key is in contact.

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