Luminous Designs.
Coat one side of a glass plate with tinfoil, leaving an attached strip for connection. Shellac a piece of paper of a size corresponding to the design to be rendered luminous. When the shellac has dried so far as to become "tacky," lay a sheet of foil on it and press it down evenly all over.
Then draw on the paper a design that can be readily cut out. Use a pair of scissors or a very sharp knife. If the latter, lay the sheet on a piece of glass; but there is a greater tendency to tear the design when a knife is used if an unpractised hand wields it.
This design may either be stuck on to the plain side of the glass plate with varnish or simply laid on (Fig. 38). Connect one secondary wire to the foil coating of the plate and the other to the design. This must be shown in the dark, and the luminosity will not be strikingly apparent until the eyes become accustomed to the darkness—that is, when the room has been previously lighted.
One of the most beautiful and easily obtained phenomena of the high-tension discharge is the "electric brush" (Fig. 39). This occurs when the secondary electrodes of the coil are too far apart to allow of the free passage of the spark, and can only be seen at its best in a perfectly dark place. The ball tips before mentioned show this brush very plainly, or two sheets of tinfoil in circuit hung far enough apart to prevent vivid sparking will cause this so-called "silent" discharge. This latter arrangement should not be used for over fifteen minutes, as the ozone which is liberated in large quantities will affect those persons in the vicinity.
Fig. 38.
Fig. 39.
In fact, when a rapid vibrator is being used with the coil, the leading wires from the secondary terminals present this brush appearance, the curious threads of light resembling luminous hairs waving in the air. The more rapid the vibrations the more prominent the brush effect, as will be seen in the Tesla coils. The positive ball of the discharger shows the brush as a spreading mass of luminous threads reaching out toward the negative ball, which latter resembles a star, as in the figure.
The intensely disruptive power of the long spark is readily shown by its power to perforate substances, but great care must be taken that the secondary wires of a coil are led away from the body of the coil. A good plan is to hang two silk cords or stout threads from the ceiling, to which the secondary wires may be attached and kept in sight when experimenting at any distance from the coil.
To pierce a piece of thin glass, take two lumps of paraffin about the size of a walnut, and, warming them and the glass sheet, stick them on opposite sides of the glass facing each other. Then warm the ends of the two pointed wires and thrust them into the lumps of paraffin, that they terminate on the glass surface directly opposite each other. On connecting these to the secondary coil a few impulses to the contact breaker will start an electric discharge sufficient to pierce the glass if the thickness be proportioned to the power of the apparatus. The great Spottiswood coil pierced a block of glass 6 inches in thickness.
There is, however, a certain element of danger to the secondary insulation in performing this experiment.
CHAPTER VI.
SPECTRUM ANALYSIS.
If a metal or the salt of a metal be burned in a flame it imparts to the flame a distinctive color; table salt thrown into the fire burns with a yellowish flame, denoting the presence of sodium, and a greenish tint, indicating the combustion of chlorine. Violet flames accompany the burning of the salts of potassium, and barium burns green. Lithium and strontium give a red hue. But to be ordinarily perceptible, the salts require for the most part to be present in considerable quantities. By the use of the spectroscope, however, extremely small proportions of these metals and salts can be readily detected and classified.
Fig. 40.
If a beam of light be transmitted through a prism of glass the rays are decomposed, and what is known as a spectrum is formed (Fig. 40). The most generally observed spectrum is the rainbow. When the light from a flame in which is burning some suitable substance be transmitted through the prism, the color which predominates in the flame will predominate in its spectrum. The combination of a prism and tubes for observing these effects is a spectroscope (Fig. 41). The short fat spark from the Ruhmkorff coil is most useful in this work. The electrodes are provided with a portion of the substance to be examined, and the spark is passed and viewed through the spectroscope.
Fig. 41.
The spectroscope is shown in connection with the coil in Fig. 41. A is the aperture in the screen through which the rays from the metal burning at the discharger balls D D passes. The lens at L is used to view these rays after they have been decomposed by the prism P, which, as well as the lens, can be rotated. I is the coil, P P the primary and S S the secondary wires, C being a condenser bridged across the circuit.
The screen should be pierced by a very narrow aperture, A, and be placed at a considerable distance from the prism P, that the rays issuing through the aperture may not strike the prism until they have widely diverged and become separated from each other. The aperture is practically formed of perfectly parallel knife edges, forming a slit not exceeding one hundredth of an inch in width.
The colored spaces in the solar spectrum do not occupy an equal extent of area; the violet is the most extended, the orange the least. The proportion is in three hundred parts: Violet, 80; green, 60; yellow, 48; red, 45; indigo, 40; orange, 27.
The solar rays exhibit on careful examination dark lines crossing the spectrum at right angles to the order of the colors, and always occupying the same relative positions. These are called Fraunhofer's lines.
If, however, the spectra of metals, gases, and other elements be examined they will be found to present certain characteristic bright lines, the body of the spectrum being often feeble or entirely dark. The spectrum of hydrogen gives two very bright lines of red and orange.
An extremely minute quantity of an element is necessary to give distinct lines. Sodium gives a single or double line of yellow light in a position agreeing with that of the orange rays in the solar spectrum.
Potassium gives a red line in the red end and a violet line in the violet end of the solar spectrum. Strontium presents eight bright lines; calcium gives mainly one broad green band and one bright orange band.
In practical work with the spectroscope a solar spectrum is often arranged that it can be used as a comparison with the spectrum being investigated, one spectrum being formed above the other, and the observation made as to which lines coincide. Iron gives nearly sixty bright lines coinciding with the same number of dark lines of the solar spectrum.
The violet rays of the solar spectrum are the rays which possess the maximum chemical action, the yellow the maximum light effect, the red the maximum heating effect. Beyond the violet band of the spectrum exist certain rays termed the invisible rays or ultra-violet rays, which in themselves are not luminous. Their vibratory rate is higher and their wave length shorter than the violet rays, according to the most generally accepted theory of light. These rays, when passed through certain substances, suffer a change and become visible in a luminous state of the substance, which luminosity is termed fluorescence.
The bright yellow line of sodium in the orange rays is found in nearly all spectra, owing to its extensive diffusion in the atmosphere.
Tesla has succeeded in producing electric waves of length approximating to those of white light, which appear to have very little heat. The ideal light is that which shows no heat and does not liberate noxious gases in the air, and were it not for its feeble luminosity, the light of the electric spark passing through a carbonic acid vacuum would approximate this most nearly.
The present mode of obtaining light—that of raising to a high temperature some substance or collection of particles—seems certainly somewhat antiquated. The following notes may be of interest and assistance in researches bearing on the lighting question.
Solid bodies, when heated, show a red glow in daylight at an elevation of temperature corresponding to 1000° Fahr.
| Temperature, degrees F. | Color of Substance. | |
|---|---|---|
| 1000 | Red. | |
| 1200 | Orange. | |
| 1300 | Yellow. | |
| 1500 | Blue. | |
| 1700 | Indigo. | |
| 2000 | Violet. | |
| 2130 | All colors—i.e., white. | |
The number of vibrations per second necessary for the production of light, and the velocity of light being determined, the calculation of the wave lengths of the colored rays becomes possible.
The following table (Sprague) shows this in ten-millionths of a millimetre (a millimetre = .039 inch) measured in the dark lines of the solar spectrum, from red to violet:
| Orange = | 6.88 |
| Orange, Higher = | 6.56 |
| Yellow = | 5.89 |
| Green = | 5.26 |
| Blue = | 4.84 |
| Blue, Higher = | 4.29 |
| Violet = | 3.93 |
CHAPTER VII.
CURRENTS IN VACUO.
Notwithstanding it requires an intensely high potential to enable the current to jump an air gap of 1 inch, the same potential will produce a luminous discharge through exhausted glass tubes aggregating 8 feet or even more.
But the exhaustion can be carried so far that there is no apparent discharge; and, on the contrary, air at as high a pressure as 600 pounds per square inch will resist the passage of the spark over an extremely short space. If the tubes be filled with various gases and then partially exhausted, the length of tube through which the luminous discharge will pass varies with the gas, becoming shorter in the following order: Hydrogen, nitrogen, air, oxygen, and carbonic acid—the shortest.
Fig. 42.
Before detailing some of the more striking phenomena connected with high-tension discharges in vacuo, a description of a few forms of simple mercurial air pumps will be serviceable.
Fig. 42: If a glass tube, F, stopped at one end, 3 feet long or over, be filled with mercury and the open end immersed in a vessel of mercury, T, the column of metal in the tube will sink until it attains a height, M, of about 30 inches, varying according to the condition of the atmosphere.
The space between the mercury column and the top of the tube will be a fairly good vacuum. This fact was noted many years ago, and the gradual evolution of the mercurial air-pump based on this result can be followed in the articles on the mercurial air-pump by Silvanus P. Thompson, read before the Society of Arts, England, some years ago.
Geissler, the first manufacturer of the "Geissler" or vacuum tube for electrical research, seeing the inconvenience of the above-described operation and the meagre results obtained, invented the pump called by his name (Fig. 43).
F E is a stout glass tube some 3 feet long, having a bulb, B, at its upper extremity, and a rubber tube, S, attached to the curved end. A reservoir of mercury, R, connects with this rubber tube, and a special glass tap is fixed in the upper end of the glass tube at E, beyond which tap being the point of attachment for the object to be exhausted. The operation is as follows: On turning the tap a part of the way it allows a passage between the tube F E and the atmosphere. The reservoir R is then raised until the mercury flows into the bulb and up the tube to the tap. The tap is then turned a fraction, and the communication with the air is shut off and opened between the object to be exhausted and the tube F E. The reservoir is then lowered and the mercury falls, drawing down the air from the object into the tube. The tap is then turned as in the first place, and the reservoir R raised, when the air drawn into the tube is forced out by the rising column of metal. This operation being repeated many times, withdraws nearly all the air from the object—in fact, makes a fairly good vacuum. This pump has been much modified from the simple form described.
The form of pump most used in the United States lamp factories is based on the application of the piston-like action of a quantity of mercury dropping down a tube. This is known as the Sprengel pump, after the inventor.
Fig. 43. Fig. 44.
Fig. 44: F is a stout glass tube about 40 inches long by one-twelfth of an inch internal diameter, carrying the reservoir funnel R at the top, a piece of soft rubber tubing, S, nipped by a pinch-cock being interposed to admit of the regulation of the mercurial drops. The lower end of this "fall tube," as it is called, is immersed in mercury contained in a suitable vessel, V, a branch tube being blown or cemented into the fall tube to admit of the connection of the object to be exhausted at E. S is another piece of rubber tubing with a pinch-cock regulation. The point H is the normal barometric height of the mercury—about 30 inches. On attaching a bulb, for example, at E, and regulating the pinch-cock at the top of the fall tube F, a succession of drops of mercury falls down the tube, each drop acting as a piston to drive the air before it, sucking the same from the bulb, and forcing it down through the tube and vessel out into the atmosphere.
On its first being set into operation, the cushions of air between the drops silence their fall; but as a higher degree of rarefaction occurs, the air cushions become insufficient, and the drops fall with a sharp click on the top of the barometric column.
One great disadvantage in this form of pump is the tendency to fracture of the glass tube that is manifested by the concussion of the drops of mercury at the barometric height. However, this has to a certain extent been obviated in later forms of this useful and efficient pump.
For many electrical experiments, the simple exhaust tube (Fig. 42) mentioned at the beginning of the article will be found very satisfactory. The top end need not necessarily be sealed off with glass, a cork having a wire, W, run through for connection being driven in, and a coat of paraffin or one of the cements mentioned in a later chapter be laid on.
The second electrical connection is made by a wire dipping in the tumbler of mercury.