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FIG. 545.- Wireless Station, Wellfleet, Mass.

Many devices have been developed that will do this with more or less efficiency. One which combines sensitiveness and convenience consists of a contact between two crystals or between a metal and a crystal. Silicon, zincite, bornite, carborundum, and molybdenite have been successfully used.

The action of this type of detector depends upon the fact that the contact between the crystals offers a much greater resistance to the passage of the current in one direction than in the other. Thus one part of the electric wave will produce current through the detector and the telephone, while that part of the wave which is in the opposite direction will produce no current and is unheard.

The form of antenna used depends upon the power of the station. For a small output the straight vertical form is

generally sufficient. A flat top is sometimes used with the vertical, as in Fig. 546. For receiving, nearly any metal surface raised above the ground and insulated from it will serve as antenna.


Signals from a neighboring send

ing station may frequently be heard in the receiver of an ordinary telephone system, the wires acting as antenna and the house fuse block as detector.

FIG. 546

Daily communication between stations on opposite sides of the Atlantic and occasional messages at such distances as from the Arlington station near Washington to Honolulu,

a distance of nearly 6000 miles, show the great advances that have been made since the initial experiments of Hertz.

569. Cathode Rays. In considering the luminous effects of the inductive discharge (§ 445) we saw that the spark is longer and much more brilliant in a partial vacuum than in ordinary air. Professor William Crookes made an extensive study of the phenomena of electric discharges in high vacuum tubes. A high vacuum is one in which the gaseous pressure is not more than one millionth of that of the atmosphere. Figure 547 shows the form of tube with which he studied the difference between the phenomena when high and low vacua are used. If the vacuum is low, the discharge shows a curved band of light from the cupshaped platinum cathode to whichever FIG. 547.- Crookes Tube wire is made the terminal. If, however, the vacuum is high, the discharge passes from the cathode directly across the globe to the opposite wall, which glows with a yellowish green fluorescence.

A study of the action of the cathode rays (§ 570) leads to the conclusion that they consist of a stream of minute, negatively charged particles that are projected from the cathode in a direction perpendicular to its surface, with a velocity about one tenth that of light (Fig. 547).

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570. Effects of the Cathode Rays. (a) The Mechanical Effect. In the tube shown in Fig. 548 a light wheel is made

to roll along the glass rails, either to the left or to the right, depending on whether A or B is made the cathode. The rotation of the wheel is caused by the stream of particles sent off by the cathode, which strike the vanes on the top of the wheel. (b) The Heating Effect. The

tube shown in Fig. 549 is a focus




FIG. 548

tube having a thin piece of platinum at the focus of the cup-shaped cathode. The continuous blows of the repelled particles constituting the cathode rays cause this piece of platinum to become redhot.

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(c) The Magnetic Effect. By using a straight tube and placing a strong electromagnet near one side, as in Fig. 550, it is possible to deflect the rays from their straight path whenever an electric current is sent through the coil of the electromagnet. The fact that the rays are deflected by a magnetic field proves that they are made up of FIG. 549 charged bodies, and the direction of this deflection indicates the kind of charge carried.


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(d) The Fluorescent Effect. The tube shown in Fig. 551 contains, as the anode, a cross of aluminum or mica, hinged to a support at the bottom. When a current is

FIG. 550

sent through the tube, the bombardment of the cathode rays causes the glass walls of the tube to glow with a fluorescent light except where they are protected by the cross, which

itself receives the bombardment. This shows the straightline path of the particles. If now the cross is suddenly swung down on its hinge to the bottom of the tube, the part of the tube that was dark before will glow more brightly than the rest. This is due to the fact that the fluorescence fades out after the rays have been striking the glass for some time. The glass may be said to possess a fluorescent fatigue.


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FIG. 551

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571. The Röntgen Rays or X-Rays. Certain luminous tubes, when the secondary current from an induction coil is passed through them, send off rays that make fluorescent substances glow and affect the photographic plate. These rays have, moreover, the property of passing through many opaque substances as rays of light pass through transparent substances. They proceed from that part of the surface of the tube upon which the cathode rays strike, and were called by Professor Röntgen, of Würzburg, their discoverer, the X-rays. Unlike the cathode rays, they are not affected by a magnetic field. Unlike light waves, they are not refracted by lenses nor reflected by mirrors. They may perhaps best be described as irregular pulses set up in the ether as a result of the impact of the cathode rays upon the side of the tube.

572. Radiographs. While the X-rays pass through flesh with very little difficulty, the bones offer much obstruction. This makes it possible to locate the position of the bones by means of X-ray photographs, or radiographs (Fig. 552). The

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