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Fig. 95.

ELECTRICITY

1165 which expand when heated, and thermo-electric | trodes is so strikingly different that the discharge couples. "Lodge has recently brought out the loses all appearance of uniformity. Around the most sensitive detector of electric waves; it consists of a glass or paper tube filled with coarse iron filings. This tube is placed in circuit with a battery and galvanometer, the galvanometer showing a deflection depending upon the resistance of the filings: but if the electrical radiation is allowed to fall on the tube, the resistance of the filing, is greatly reduced, thus causing a large increase in the deflection of the galvanometer. This detector is extremely sensitive, and is capable of detecting the electric radiation when placed at a distance of several hundred feet from the oscillator. The form of the oscillator has lately been very much improved; it often consists of two small spheres, a centimeter in diameter or negative electrode or cathode there is a dark smaller, immersed in oil. The oil prevents the space, and then alternate bright and dark bands, balls from tarnishing, as it is necessary to keep the size and number of which depend on the dethe surfaces between which the gap is formed gree of rarefication of the gas. continually bright. The balls are made smaller, since they will then have a smaller capacity, and the oscillations a correspondingly high fre

+ quency, thus giving a short wave-length.

94. DISCHARGE AT High POTENTIAL. As the potential of the current causing a disruptive discharge increases, the nature of the discharge changes its appearance. At first the spark is a thin bright line between the terminals; this line grows broader (the coil furnishing a high potential current) and becomes more like a flame as the potential of the current increases, and finally,

Fig. 96. at very high potential, breaks into numbers of Fig. 96 represents the appearance of the dissmall sparks, which shoot out in all directions charge under these conditions. The exhausted from each terminal, totally irrespective of the rel. tube is known as a Geisler tube. Crookes found

ative positions of the two. When this that if the exhaustion be continued, the dark point is reached, an exhausted bulb, space about the cathode increased in width, and such as a Crookes tube, will light up that across the space electrified molecules were when attached to only one (it is im- projected in lines perpendicular to the surface of material which one) of the terminals. the electrode. If the exhaustion be pushed alIf two long wires, one end of each most to the limit, the dark space fills the entire attached to each terminal of the tube, and the streams of electrified molecules, or generator, be stretched out parallel cathode rays as they are termed, impinge against to each other (Fig. 94), the space be the glass wall of the tube, causing the glass to tween will be filled with showers of emit a beautiful fluorescent light, the color define sparks, so that the whole has pending upon the nature of the glass; the tube the appearance of a band of soft is then known as a Crookes tube. Hard German light. If two metal plates (Fig. 95) glass gives a greenish yellow light, probably due be set up parallel to each other, sev- to the small amount of uranium it contains, while eral feet apart, and each connected lead glass fluoresces with a pale blue light. . A

with one terminal of the generator, solid object placed in the path of the cathode there will be no apparent discharge, but an ex- rays protects portions of the glass, thus casting hausted bulb, when brought into the space be a shadow, the fluorestween the plates, will glow with a soft white light. cence only appearing If the potential of the discharge is high enough, on the glass where the

N it can be taken through the body, not only with cathode rays strike. out harm, but without sensation of any kind. Fig. 97 shows a shadThe discovery of these, and many other beautiful ow tube; a is called effects of high potential discharges, and the in- the cathode, b is a vention of the simplest means of producing them, plate of metal mounted are due to Tesla. (See Tesla Coil, $ 75.)

inside the tube and in 8 95. DISCHARGE IN High VACUA—CATHODE the path of the rays; Rays. When a disruptive discharge passes be- d is the portion of the tween electrodes in a rarefied gas, the appearance glass protected from the cathode rays by the of the discharge at the positive and negative elec- screen, b. Diamonds, rubies, many crystals and

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Fig. 94.

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Fig. 97

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salts, when placed in the path of the cathode | inversely propositional to the density of the streams, become brilliantly fluorescent (Fig. 98). substance; that they cause fluorescence when Calcium tungstate, under these conditions, emits a allowed to fall on certain substances, and are very brilliant bluish white light; it has been propos capable of acting on a photographic plate; that

ed to utilize this light for an electrified body loses its charge when these purposes of illumination. rays are allowed to fall upon it; that they were

Cathode rays develop deflected by a magnet always in a vacuum, and peeeee heat when they impinge sometimes in air. Lenard considered these rays,

against a substance; the which he called cathode rays in air, an ethereal

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be greatly phenomenon, and not charged particles of gas.

magnified by making the The late discoveries of Roentgen and other phyFig. 98.

cathode a concave plate, sicists, working along the line he proposed, have

as shown in Fig. 99. The added greatly to our knowledge of these pherays are thus concentrated on a platinum plate, nomena. P, which becomes intensely heated and is incan- $ 96. ROENTGEN RAYS AND PHOTOGRAPHY. Prodescent.

fessor Roentgen, in 1895, made the startling disThe rays are attracted by a magnet, and exert covery that when the cathode rays inside a

a force upon the object against Crookes tube impinge on the glass they origi. which they strike. This is nate a new form of radiation hitherto unknown. shown by the tube in Fig. 100; These he termed, for brevity, “X rays," on the cathode is concave, and con- account of their unknown character; however,

centrates the rays on the it is but just that they should be termed “RoentP.

screen d. A magnet placed at gen rays," in honor of the discoverer. Roentgen, m deflects the rays so that they in his original paper, ascribes to these rays the strike against the veins of the following properties. They excite fluorescence pivotal wheel, causing it to ro- in many substances, the most sensitive being tate.

barium platino-cyanide, which showed fluoresIf, in these tubes, the exhaus- cence when placed at a distance of two meters tion is pushed to the extreme from the tube; a screen of cardboard covered limit, the discharge no longer with this salt formed the detector used by takes place through the tube, Roentgen in many of his experiments. The rays going, in preference, through possess the property of penetrating substances many centimeters of air; hence opaque to light, such as a book of one thousand the electric discharge cannot pages, thick blocks of wood, aluminium, 15 m.m. pass through a vacuum.

(approximately 9.16 inches) thick, thick glass plates, ebonite several centimeters thick; and the hand, if held before the fluorescent screen (between it and the tube), shows only a shadow of the bones, since the flesh allows the rays to pass almost unobstructed, while the bones are opaque.

It is this property of the Roentgen rays which has Fig. 100.

interested the public more than any other scien

tific discovery of late years. Many important Crookes, who devised these tubes and many applications of it have been made in surgery, others, attributed the phenomena to streams and time will no doubt develop many more. of charged gaseous molecules repelled from the Water and several other liquids are very transcathode, but their exact nature is yet a matter of parent, but the metals are in general opaque. doubt.

Plates of gold, silver, copper, platinum and lead In 1892 Hertz discovered that thin films of gold allow the rays to pass, but only when the plates are are somewhat transparent to these cathode rays, thin; the opacity of bodies for these rays seems to -a discovery that led him to believe he could depend only on their density. The rays are not get them outside the tube, and so study their refracted or regularly reflected to any great exproperties in air. This was actually accomplished tent, if at all, nor do they give rise to interby Lenard the following year. Lenard succeeded ference or polarization phenomena; in these rein getting the cathode rays outside his exhausted spects the radiation is totally unlike ordinary receiver through a small, thin window of alumin- | light. ium foil. He discovered that in air the cathode Photographic dry plates are sensitive to the rays are not a prolongation of the cathode stream rays, and, as a consequence, a new photography has in the vacuum tube, but that they spread out been developed, in which the object is depicted acdiffusely from the window; that they differ from cording to the transparency of its parts to these light in their penetrating power, being able to rays, while an ordinary photograph records the pass through aluminium, but not through glass. reflection of ordinary light from the surface of He concluded that they were not electrical rays, objects. Fig. 101 shows the manner in which as conductors were not opaque to them; he also photographs are made with Roentgen rays. found that their penetrating power is roughly The Crookes tube, T, is supported in any con.

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Fig. 99.

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venient manner, as a stand, S; the primary of the tube, the negative terminal of the coil being coninduction coil, D, is connected to three or four nected with the cathode of the tube. The dry

plate is inclosed in a light-tight case or holder, C, which is placed directly below the tube, and the hand or other object to be photographed is placed on the holder, as shown. When the circuit is made through the primary of the coil, the automatic vibrator, V, interrupts the current several times per second, and the secondary of the coil discharges through the tube at each interruption. The tube emits a pale greenish yellow light where the cathode rays strike, but the flourescent light is not to be confused with the Roentgen rays, which are invisible. The time of exposure depends upon the thickness or density of the body to be photographed and the intensity of the radiation, which depends largely on the character of the tube and the current sent through

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Fig. 103.

it. Slow plates appear to be about as sensitive as quick ones.

Figs. 102 and 103 show photographs taken in this manner.

While the whole scientific world has, to a great extent, been interested in this discovery, and many of the ablest investigators have attempted to solve this intensely fascinating problem, very little has been added to our knowledge as to the true nature of the Roentgen rays.

Lenard's cathode rays in air must have been in part at least, Roentgen rays, since the condition under which he worked and the tests applied

Fig. 102.

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Fig. 105.

would produce and detect Roentgen rays. Whether | mersed in dilute sulphuric acid, it constitutes or not Lenard's rays and Roentgen's are dif- the simplest and the typical voltaic cell. (See ferent modifications of the same thing remains to Fig. 105.) When the plates be determined.

are placed in the acid, bubbles It has been conclusively demonstrated that the of hydrogen collect on the source of the Roentgen rays is the first dense ob- zinc, but the chemical action ject the cathode rays strike. In the vacuum this soon ceases. If, however, the object need not be the wall of the tube, but may plates are connected by a conbe any solid placed in the path of the cathode ductor, a current will flow streams. Advantage is taken of this fact in con- around the circuit, flowing from structing tubes for photographic work. If very zinc to copper through the acid, powerful streams are directed against the glass, and from copper to zinc in the it becomes heated and breaks down. A piece of conductor. The acid attacks platinum placed in the path, since it can with the zinc, forming zinc sulphate, while hydrogen stand great heat, permits of a concentration of is freely liberated, and collects at the surface of the cathode rays, hence a powerful source of the copper plate. For each unit of electricity Roentgen rays.

that flows around the surface one electro-chemFig. 104 is a Crookes tube constructed on this ical equivalent of zinc and sulphuric acid disapprinciple; these are called focus tubes, but it is to pears, and equivalent amounts of zinc sulphate

be understood and hydrogen are formed. The E.M.F. has that there is no already been defined as the work done by unit quanfocusing of the tity of electricity in passing entirely around the cir

Roentgen rays.cuit, and it is interesting to note that the E.M.F. M'

The concave can be predicted from the principle of conservacathode, M, tion of energy; for the work done by the current concentrates appears as heat in the circuit, which we have seen the cathode is I ́R, where I is the strength of current and R rays upon the the resistance of the circuit. It is also known

piece of plati- that when a current flows across the juncture of Fig. 104.

num, M'; here two conductors, heat is developed; hence in a the cathode rays are totally or in part changed simple cell the heat appears in three quantities, into Roentgen rays, which radiate in all directions as follows: That developed in the outside circuit from the front of the platinum plate. One ad

One ad- of resistance, R; that developed in the cell, due vantage in this form is, that the rays, coming to the internal resistance of the cell, r; and that from a small source, produce sharp photographs developed at the juncture, which can be repreor shadows on the fluorescent screen.

sented by H. Then if a current, I, flows for a Many of the properties of Roentgen rays lead time, t, the number of units of electricity that us to believe that the radiation consists of short have passed around the circuit in the time is It. waves in the ether, shorter than the waves of violet | If e is the electro-chemical equivalent of zinc (the light; other properties indicate a long wave-length, amount of zinc that combines with sulphuric acid longer than the waves of red light. Roentgen sug- to produce unit quantity of electricity (see Elecgests that they are longitudinal waves in the ether, TROLYsis, in these Supplements), Ite is the number while others insist that it is not a wave-motion of grams of zinc consumed in the time, t. If W is the at all, though the latter view has few adherents. work-equivalent of the heat produced when one

The solution of the problem is looked forward gram of zinc combines with sulphuric acid to proto with great interest, both by the scientific and duce zinc sulphate, then the work-equivalent of the unscientific world.

heat which would be developed by the chemical ac$ 97. PRIMARY CELLS. A primary or voltaic | tion of the cell in the time, t, is IteW, which must be cell is a device for producing electrical energy equal to the heat developed by the current. Hence in the form of a continuous current at the ex- we may write pense of chemical energy. It consists usually

I'Rt + I'rt+ HIt= IteW, of two plates immersed in an electrolyte. (See

or I(R +r) = eW-H, ELECTROLYsis, in these Supplements). The elec

eW-H and I =

; trolyte combines chemically with at least one of the

(R+r) plates, and when the combination is complete the therefore eW H is the E.M.F. of the cell. electrical action ceases. Such a cell is termed a The simple form of cell does not give a conprimary cell. If, however, after the chemical stant E. M.F., as some of the hydrogen produced combination is complete, the resulting substances adheres to the copper plate, forming a gaseous may be decomposed by an electric current, and film, which increases the resistance of the cell, thus put back into the original form, the cell is and also acts as a reverse E.M.F. The copper called a secondary cell, which is but another name plate in this condition is said to be polarised, for accumulator or storage-cell. (See SECONDARY and is the seat of a reverse E.M.F. which CELLS, $ 109).

reduces the theoretical E.M.F. in value. The $ 98. TYPICAL VOLTAIC CELL. If a strip of reverse E.M. F. may be explained in the following amalgamated zinc and one of pure copper be im- manner: The hydrogen follows the current, and

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its atoms carry positive charges; these, arriving at while the zinc and zinc sulphate are at the top (see the copper plate, are prevented from coming in con-Fig. 107). Such cells are called gravity cells. The tact at once with the plate by the film of gas already solutions are also sometimes kept separate by a there. The layer of atoms in this condition may be porous cup, which extends but half way down the considered to form one plate of a condenser and jar, the bottom of the cup forming the partition. the copper plate the other, and this condenser tends g 100. CALCULATION OF THE E.M.F. OF to discharge around through the liquid in the re- DANIELL CELL. When one electrochemical equivverse direction to that in which the current flows. alent of zinc sulphate is formed at the zinc plate,

Polarization is usually prevented by chemical one electrochemical equivalent of sulphuric acid means. In the Daniell cell and similar cells, the disappears; but at the copper plate one electrohydrogen is utilized in the action of the cell; in chemical equivalent of sulphuric acid is formed, other cases, substances are placed in the cell, and one electro-chemical equivalent of copper which combine with the hydrogen or oxidize as sulphate disappears. Hence the source of elecsoon as formed. Many different primary cells trical energy is the chemical energy lost when have been devised, but since they are all modifi- the copper in the copper sulphate is replaced cations of a few principal forms, which may be by zinc, which is equivalent to the difference taken as types, the latter only will be described. between the heat developed when zinc dis.

$ 99. THE DANIELL Cell. The Daniell cell is solves in sulphuric acid and that developed one of the oldest and best known, and may be when an equivalent amount of copper dissolves taken as the type of all cells in which zinc and in sulphuric acid. When one gram of zinc discopper and copper sulphate are used. In the solves in sulphuric acid the heat developed is

Daniell cell (Fig. 106) the 1670 thermal units, or 1670 X 4.2 X 10' work-units, 2

plates are zinc and cop-4.2 X 10' being the work-equivalent of one therper. The zinc rod, Z, is mal unit, or the mechanical equivalent of heat. placed in a porous cup, P, The electro-chemical equivalent of zinc is with dilute sulphuric | 0.003364 grams; hence the heat developed when acid, but later the liquid one electrochemical equivalent of zinc is disin the porous cup be solved in sulphuric acid is 0.003364 X 1670 X 4.2

zinc sulphate. X 10' = 2.359 X 10% work-units. When one gram The copper plate, C, sur- of copper is dissolved in sulphuric acid, 909.5 rounds the porous cup; thermal units are developed, which are equivalent the space between the to 909.5 X 4.2 X 10' work-units. The electro. cup and jar is filled chemical equivalent of copper is 0.003261, hence with a saturated solution the heat developed when one electro-chemical of copper sulphate, with equivalent of copper is dissolved is 0.003261 X crystals of the same salt-| 909.5 X 4.2 X 10'=1.245 X 108 work units. to replace that in solu- The difference between these two quantities,

tion as fast as it used up 1.114 X 10°, must represent the work done by one Fig. 106.

in the action of the cell. unit of electricity in going around the circuit, or The sulphuric acid acting upon the zinc forms the E.M.F. of the cell. The E.M. F. of a Daniell zinc sulphate and free hydrogen; the hydrogen, cell is found to be approximately 1.028 X 10%; the traveling with the current, passes through the difference between this and the calculated result porous cup to the copper sulphate, with which it is insignificant, and is probably due to the quantity combines, forming sulphuric acid, and setting free of heat, H, of $ 98. pure copper, which also travels with the current, § 101. Grove's Cell. Grove's cell is a zinc

and is deposited on platinum cell; the zinc is acted upon by sul-
the copper plate. phuric acid, while the platinum is placed in a porous
The hydrogen, which cup containing strong nitric acid, which oxidizes
ordinarily is detri- the hydrogen and
mental to the action prevents polariza-

C.

Z
of the cell, becomes tion. Since there is
useful in keeping up

no chemical action
the supply of acid to upon the platinum,
act upon the zinc. the electromotive
The cell may be set force is high, being
up with the zinc sul- approximately
phate solution in the volts.
cup, instead of dilute § 102. BUNSEN'S
acid. Since the cop- CELL. The Bunsen
per sulphate is heav-cell (Fig. 108) is like
ier than the zinc sul- | the Grove,

the Grove, except
phate, the solutions that the platinum is
Fig. 107.
are often kept sep- replaced by hard

Fig. 108. arate by the action of gravity, the copper plate carbon, which, of course, is more economical. and copper sulphate being in the bottom of the jar, | The cell may be modified by placing the zinc and

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