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 smaller, immersed in oil. The oil prevents the balls from tarnishing, as it is necessary to keep the surfaces between which the gap is formed continually bright. The balls are made smaller, since they will then have a smaller capacity, and the oscillations a correspondingly high frequency, thus giving a short wave-length. 894. 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, at very high potential, breaks into numbers of small sparks, which shoot out in all directions from each terminal, totally irrespective of the rel Fig. 95. negative electrode or cathode there is a dark space, and then alternate bright and dark bands, the size and number of which depend on the degree of rarefication of the gas. + Fig. 96. Fig. 96 represents the appearance of the discharge under these conditions. The exhausted tube is known as a Geisler tube. Crookes found that if the exhaustion be continued, the dark space about the cathode increased in width, and that across the space electrified molecules were projected in lines perpendicular to the surface of the electrode. If the exhaustion be pushed almost to the limit, the dark space fills the entire tube, and the streams of electrified molecules, or cathode rays as they are termed, impinge against the glass wall of the tube, causing the glass to emit a beautiful fluorescent light, the color depending upon the nature of the glass; the tube Hard German is then known as a Crookes tube. glass gives a greenish yellow light, probably due to the small amount of uranium it contains, while lead glass fluoresces with a pale blue light. A solid object placed in the path of the cathode rays protects portions of the glass, thus casting a shadow, the fluorescence only appearing on the glass where the cathode rays strike. ative positions of the two. When this point is reached, an exhausted bulb, such as a Crookes tube, will light up when attached to only one (it is immaterial which one) of the terminals. If two long wires, one end of each attached to each terminal of the generator, be stretched out parallel to each other (Fig. 94), the space between will be filled with showers of fine sparks, so that the whole has the appearance of a band of soft light. If two metal plates (Fig. 95) be set up parallel to each other, several feet apart, and each connected Fig. 94. with one terminal of the generator, there will be no apparent discharge, but an exhausted bulb, when brought into the space between the plates, will glow with a soft white light. If the potential of the discharge is high enough, it can be taken through the body, not only without harm, but without sensation of any kind. The discovery of these, and many other beautiful effects of high potential discharges, and the invention of the simplest means of producing them, are due to Tesla. (See TESLA COIL, § 75.) 895. DISCHARGE IN HIGH VACUA-CATHODE RAYS. When a disruptive discharge passes be-d is the portion of the tween electrodes in a rarefied gas, the appearance of the discharge at the positive and negative elec Fig. 97 shows a shadow tube; a is called the cathode, b is a plate of metal mounted inside the tube and in the path of the rays; Fig. 97. a N glass protected from the cathode rays by the salts, when placed in the path of the cathode streams, become brilliantly fluorescent (Fig. 98). Calcium tungstate, under these conditions, emits a very brilliant bluish white light; it has been proposed to utilize this light for purposes of illumination. Cathode rays develop heat when they impinge against a substance; the effect can be greatly magnified by making the cathode a concave plate, as shown in Fig. 99. The rays are thus concentrated on a platinum plate, P, which becomes intensely heated and is incandescent. Fig. 98. In 1892 Hertz discovered that thin films of gold are somewhat transparent to these cathode rays, -a discovery that led him to believe he could get them outside the tube, and so study their properties in air. This was actually accomplished by Lenard the following year. Lenard succeeded in getting the cathode rays outside his exhausted receiver through a small, thin window of aluminium foil. He discovered that in air the cathode rays are not a prolongation of the cathode stream in the vacuum tube, but that they spread out diffusely from the window; that they differ from light in their penetrating power, being able to pass through aluminium, but not through glass. He concluded that they were not electrical rays, as conductors were not opaque to them; he also found that their penetrating power is roughly | inversely propositional to the density of the substance; that they cause fluorescence when allowed to fall on certain substances, and are capable of acting on a photographic plate; that an electrified body loses its charge when these rays are allowed to fall upon it; that they were deflected by a magnet always in a vacuum, and sometimes in air. Lenard considered these rays, which he called cathode rays in air, an ethereal phenomenon, and not charged particles of gas. The late discoveries of Roentgen and other physicists, working along the line he proposed, have added greatly to our knowledge of these phe nomena. $96. ROENTGEN RAYS AND PHOTOGRAPHY. Professor Roentgen, in 1895, made the startling discovery that when the cathode rays inside a Crookes tube impinge on the glass they origi nate a new form of radiation hitherto unknown. These he termed, for brevity, "X rays," on account of their unknown character; however, it is but just that they should be termed "Roentgen rays," in honor of the discoverer. Roentgen, in his original paper, ascribes to these rays the following properties: They excite fluorescence in many substances, the most sensitive being barium platino-cyanide, which showed fluorescence when placed at a distance of two meters from the tube; a screen of cardboard covered with this salt formed the detector used by Roentgen in many of his experiments. The rays possess the property of penetrating substances opaque to light, such as a book of one thousand pages, thick blocks of wood, aluminium, 15 m.m. (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 interested the public more than any other scientific discovery of late years. Many important applications of it have been made in surgery, and time will no doubt develop many more. Water and several other liquids are very transparent, but the metals are in general opaque. Plates of gold, silver, copper, platinum and lead allow the rays to pass, but only when the plates are thin; the opacity of bodies for these rays seems to depend only on their density. The rays are not refracted or regularly reflected to any great extent, if at all, nor do they give rise to interference or polarization phenomena; in these respects the radiation is totally unlike ordinary light. Photographic dry plates are sensitive to the rays, and, as a consequence, a new photography has been developed, in which the object is depicted according to the transparency of its parts to these rays, while an ordinary photograph records the reflection of ordinary light from the surface of objects. Fig. 101 shows the manner in which photographs are made with Roentgen rays. The Crookes tube, T, is supported in any con ELECTRICITY venient manner, as a stand, S; the primary of the induction coil, D, is connected to three or four B 1167 tube, the negative terminal of the coil being connected 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 Fig. 102. 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 would produce and detect Roentgen rays. Whether or not Lenard's rays and Roentgen's are different modifications of the same thing remains to be determined. It has been conclusively demonstrated that the source of the Roentgen rays is the first dense object the cathode rays strike. In the vacuum this object need not be the wall of the tube, but may be any solid placed in the path of the cathode streams. Advantage is taken of this fact in constructing tubes for photographic work. If very powerful streams are directed against the glass, it becomes heated and breaks down. A piece of platinum placed in the path, since it can withstand great heat, permits of a concentration of the cathode rays, hence a powerful source of Roentgen rays. Fig. 104 is a Crookes tube constructed on this principle; these are called focus tubes, but it is to be understood that there is no focusing of the Roentgen rays. The concave cathode, M, Fig. 104. concentrates the cathode rays upon the piece of platinum, M'; here the cathode rays are totally or in part changed into Roentgen rays, which radiate in all directions from the front of the platinum plate. One advantage in this form is, that the rays, coming from a small source, produce sharp photographs or shadows on the fluorescent screen. Many of the properties of Roentgen rays lead us to believe that the radiation consists of short waves in the ether, shorter than the waves of violet light; other properties indicate a long wave-length, longer than the waves of red light. Roentgen suggests that they are longitudinal waves in the ether, while others insist that it is not a wave-motion at all, though the latter view has few adherents. The solution of the problem is looked forward to with great interest, both by the scientific and unscientific world. $97. PRIMARY CELLS. A primary or voltaic cell is a device for producing electrical energy in the form of a continuous current at the expense of chemical energy. It consists usually of two plates immersed in an electrolyte. (See ELECTROLYSIS, in these Supplements). The electrolyte combines chemically with at least one of the plates, and when the combination is complete the electrical action ceases. Such a cell is termed a primary cell. If, however, after the chemical combination is complete, the resulting substances may be decomposed by an electric current, and thus put back into the original form, the cell is called a secondary cell, which is but another name for accumulator or storage-cell. (See SECONDARY CELLS, § 109). $ 98. TYPICAL VOLTAIC CELL. If a strip of amalgamated zinc and one of pure copper be im Fig. 105. mersed in dilute sulphuric acid, it constitutes the simplest and the typical voltaic cell. (See Fig. 105.) When the plates are placed in the acid, bubbles of hydrogen collect on the zinc, but the chemical action soon ceases. If, however, the plates are connected by a conductor, a current will flow around the circuit, flowing from zinc to copper through the acid, and from copper to zinc in the conductor. The acid attacks the zinc, forming zinc sulphate, while hydrogen is freely liberated, and collects at the surface of the copper plate. For each unit of electricity that flows around the surface one electro-chemical equivalent of zinc and sulphuric acid disappears, and equivalent amounts of zinc sulphate and hydrogen are formed. The E.M.F. has already been defined as the work done by unit quantity of electricity in passing entirely around the circuit, and it is interesting to note that the E. M. F. can be predicted from the principle of conservation of energy; for the work done by the current appears as heat in the circuit, which we have seen is I'R, where I is the strength of current and R the resistance of the circuit. It is also known that when a current flows across the juncture of two conductors, heat is developed; hence in simple cell the heat appears in three quantities, as follows: That developed in the outside circuit of resistance, R; that developed in the cell, due to the internal resistance of the cell, r; and that developed at the juncture, which can be represented by H. Then if a current, I, flows for a time, t, the number of units of electricity that have passed around the circuit in the time is It. If e is the electro-chemical equivalent of zinc (the amount of zinc that combines with sulphuric acid to produce unit quantity of electricity (see ELECTROLYSIS, in these Supplements), Ite is the number of grams of zinc consumed in the time,t. If W is the work-equivalent of the heat produced when one gram of zinc combines with sulphuric acid to produce zinc sulphate, then the work-equivalent of the heat which would be developed by the chemical action of the cell in the time, t, is IteW, which must be equal to the heat developed by the current. Hence we may write = I2Rt+ I'rt + HIt IteW, or I(R+r) eW--H, and I = = eW-H (R+r) therefore eW-H is the E. M. F. of the cell. The simple form of cell does not give a constant E. M. F., as some of the hydrogen produced adheres to the copper plate, forming a gaseous film, which increases the resistance of the cell, and also acts as a reverse E. M. F. The copper plate in this condition is said to be polarized, and is the seat of a reverse E. M.F. which reduces the theoretical E. M. F. in value. The reverse E. M. F. may be explained in the following manner: The hydrogen follows the current, and P Z H per. ELECTRICITY its atoms carry positive charges; these, arriving at the copper plate, are prevented from coming in contact at once with the plate by the film of gas already there. The layer of atoms in this condition may be considered to form one plate of a condenser and the copper plate the other, and this condenser tends to discharge around through the liquid in the reverse direction to that in which the current flows. Polarization is usually prevented by chemical means. In the Daniell cell and similar cells, the hydrogen is utilized in the action of the cell; in other cases, substances are placed in the cell, which combine with the hydrogen or oxidize as soon as formed. Many different primary cells have been devised, but since they are all modifications of a few principal forms, which may be taken as types, the latter only will be described. 899. THE DANIELL CELL. The Daniell cell is one of the oldest and best known, and may be taken as the type of all cells in which zinc and copper and copper sulphate are used. In the Daniell cell (Fig. 106) the plates are zinc and copThe zinc rod, Z, is placed in a porous cup, P, with dilute sulphuric acid, but later the liquid in the porous cup becomes zinc sulphate. The copper plate, C, surrounds the porous cup; the space between the cup and jar is filled with a saturated solution of copper sulphate, with crystals of the same salt to replace that in solution as fast as it used up in the action of the cell. The sulphuric acid acting upon the zinc forms zinc sulphate and free hydrogen; the hydrogen, traveling with the current, passes through the porous cup to the copper sulphate, with which it combines, forming sulphuric acid, and setting free pure copper, which also travels with the current, and is deposited on the copper plate. The hydrogen, which ordinarily is detrimental to the action of the cell, becomes useful in keeping up the supply of acid to act upon the zinc. The cell may be set up with the zinc sulphate solution in the cup, instead of dilute acid. Since the copper sulphate is heavier than the zinc sulphate, the solutions are often kept separate by the action of gravity, the copper plate and copper sulphate being in the bottom of the jar, Fig. 106. Fig. 107. 1169 while the zinc and zinc sulphate are at the top (see Fig. 107). Such cells are called gravity cells. The solutions are also sometimes kept separate by a porous cup, which extends but half way down the jar, the bottom of the cup forming the partition. § 100. CALCULATION OF THE E.M.F. OF A DANIELL CELL. When one electrochemical equivalent of zinc sulphate is formed at the zinc plate, one electrochemical equivalent of sulphuric acid disappears; but at the copper plate one electrochemical equivalent of sulphuric acid is formed, and one electro-chemical equivalent of copper sulphate disappears. sulphate disappears. Hence the source of electrical energy is the chemical energy lost when the copper in the copper sulphate is replaced by zinc, which is equivalent to the difference between the heat developed when zinc dis solves in sulphuric acid and that developed when an equivalent amount of copper dissolves in sulphuric acid. When one gram of zinc dissolves in sulphuric acid the heat developed is 1670 thermal units, or 1670 X 4.2 X 10' work-units, 4.2 X 10' being the work-equivalent of one thermal unit, or the mechanical equivalent of heat. The electro-chemical equivalent of zinc is 0.003364 grams; hence the heat developed when one electrochemical equivalent of zinc is dissolved in sulphuric acid is 0.003364 X 1670 X 4.2 X 102 = 2.359 X 108 work-units. When one gram of copper is dissolved in sulphuric acid, 909.5 thermal units are developed, which are equivalent to 909.5 X 4.2 X 10' work-units. The electrochemical equivalent of copper is 0.003261, hence the heat developed when one electro-chemical equivalent of copper is dissolved is 0.003261 X 909.5 X 4.2 X 10' = 1.245 X 10° work units. The difference between these two quantities, 1.114 X 10, must represent the work done by one unit of electricity in going around the circuit, or the E. M. F. of the cell. The E. M. F. of a Daniell cell is found to be approximately 1.028 × 103; the difference between this and the calculated result is insignificant, and is probably due to the quantity of heat, H, of § 98. § 101. GROVE's Cell. Grove's cell is a zincplatinum cell; the zinc is acted upon by sulphuric acid, while the platinum is placed in a porous cup containing strong nitric acid, which oxidizes the hydrogen and prevents polarization. Since there is no chemical action upon the platinum, the electromotive force is high, being approximately volts. 1.9 $102. BUNSEN'S CELL. The Bunsen cell (Fig. 108) is like the Grove, except that the platinum is replaced by hard Fig. 108. carbon, which, of course, is more economical. The cell may be modified by placing the zinc and |