Radiometer

“ One of his ingenious expedients was to turn to use an ordinary radiometer of large bulb, and, having fitted this with tin-foil electrodes, he found that he was able to get strongly marked shadow pictures. This application of the Röntgen principle will commend itself to many students who, being unable to provide themselves with the rare and expensive Crookes tubes, may buy a radiometer which will serve their purpose excellently in any laboratory supply store, the cost being only a few dollars, while the application of the tin foil electrodes is perfectly simple. ”

“ In the hands of many the radiometer is now only a curiosity and a toy, yet to the physicist it is an instrument of great interest as displaying a new aspect of dynamical phenomena, and may help to explain still further the nature of the radiant forces, and perhaps throw light upon other questions. It is attracting much attention from scientific men, who may be expected in due time to report the results of their own reflections and experiments upon the subject. ”

“ When heat is cut off by allowing the radiation to pass through a thick plate of alum, the velocity of rotation is somewhat slower, and when only dark heat is allowed to fall on the arms (as from a vessel of boiling water) no rotation whatever is produced.
In all respects, therefore, it is seen that the radiometer gives indications in strict accordance with theory. ”

“ The best pressure for this electrical radiometer is a little beyond that at which the dark space round the negative pole extends to the sides of the glass bulb. When the pressure is only a few millimetres of mercury, on passing the induction current a halo of velvety violet light forms on the metallic side of the vanes, the mica side remaining dark. As the pressure diminishes, a dark space is seen to separate the violet halo from the metal. ”

“ THEORY OF THE RADIOMETER.
Prof. Crookes has at length announced a theory in explanation of the movements exhibited by the remarkable "light mill" of his invention. He says: "The evidence afforded by the experiments is to my mind so strong as almost to amount to conviction, that the repulsion resulting from radiation is due to the action of thermometric heat between the surface of the moving body and the case of the instrument, through the intervention of the residual gas. ”

“ A radiometer in which the motion of the molecules of air necessary for
rotation of the vane is produced by electrification and not by heating.
Radio-micrometer.
An instrument for detecting radiant energy of heat or light form. It
consists of a minute thermopile with its terminals connected by a wire,
the whole suspended between the poles of a magnet. A minute quantity of
heat produces a current in the thermopile circuit, which, reacted on by
the field, produces a deflection. A convex mirror reflecting light is
attached so as to move with the thermopile. ”

“ Crookes, on the movement of the floating glass case of a radiometer when the arms are fixed by a magnet, which was demonstrated to the Royal Society on March 30, 1876. Mr. Crookes has since constructed a special apparatus for measuring the vacuum. A vertical plate, instead of continuously rotating in one direction, as in the ordinary radiometer, is suspended by a glass fibre, which it twists in opposite directions alternately. ”

“ Through a strip of platinum barely 1⁄500 inch in width, and less than 1⁄5000 inch in thickness, a current of electricity flows continuously. When radiation, visible or invisible, on occasion from a star, falls upon it, the strip when warmed by as little as one millionth of a [226] degree duly records the fact. An instrument, modified from the Crookes radiometer by Professor E. F. Nichols of Columbia University, New York, is more sensitive still. An exhausted hollow metal block has a window of fluorite, a mineral transparent to ether vibrations of a long range of frequencies. ”

“ To photographers the radiometer will be invaluable. As it will revolve behind the orange-colored glass used for admitting light into the so-called dark-room, it is only necessary to place one of these instruments in the window to enable the operator to see whether the light entering his room is likely to injure the sensitive surfaces there exposed ”

“ When the rays of the sun or a candle, or dark radiation from a warm body, are incident on the vanes, the dark side of each vane is repelled more than the bright side, and thus the vanes are set into rotation with accelerated speed, which becomes uniform when the forces produced by the radiation are balanced by the friction of the pivot and of the residual air in the globe. The name radiometer arose from an idea that the final steady speed of rotation might be utilized as a rough measure of the intensity of the exciting radiation. ”

“ Limb-sounder instruments will probably be capable of accommodating several radiometers for simultaneous measurements. Instruments in different spectral ranges will be employed for complementary measurements. The antenna, scanning, data handling, and power supplies should be common to any complement of radiometers used in the system. ”

“ It is probable that we shall soon have a self-acting heat-engine capable of deriving moderate amounts of energy from the ambient medium. There is also a possibility—though a small one—that we may obtain electrical energy direct from the sun. This might be the case if the Maxwellian theory is true, according to which electrical vibrations of all rates should emanate from the sun. I am still investigating this subject. Sir William Crookes has shown in his beautiful invention known as the "radiometer" ”

“ Everything on the surface of the earth, including the external surface of buildings, the grass, and leaves of trees, must be covered with an invisible deposit of radio-active material. A hill, or mountain peak, or any high mass of rock or land, concentrates the earth’s electric field at that point and consequently will receive more excited radio-activity per unit area than the plain. Elster and Geitel have pointed out that the greater ionization of the air observed in the neighbourhood of projecting peaks receives a satisfactory explanation on this view. ”

“ A discovery which gave a great impetus to the study of the radiations from active bodies was made in 1899, almost simultaneously in Germany, France, and Austria. It was observed that preparations of radium gave out some rays which were deviable by a magnetic field, and very similar in character to the cathode rays produced in a vacuum tube. The observation of Elster and Geitel that a magnetic field altered the conductivity produced in air by radium rays, led Giesel [113] to examine the effect of a magnetic field on the radiations. ”

“ While the experiments, already referred to, afford strong evidence that ordinary matter does possess the property of radio-activity to a feeble degree, it must not be forgotten that the activity observed is excessively minute, compared even with a weak radio-active substance like uranium or thorium. The interpretation of the results is complicated, too, by the presence of the radium emanation in the atmosphere, for we have seen that the surface of every body exposed to the open air must become coated with the slowly changing transformation products of the radium emanation. ”

“ Radiographs.—The radiographic action of the new radio-active bodies is very marked. However, the method of operating should be very different with polonium and radium. Polonium acts only at very short distances, and its action is considerably weakened by solid screens; it is practically annihilated by means of a screen of slight thickness (1 m.m. of glass) . Radium acts at considerably greater distances. The radiographic action of radium rays may be observed at more than 2 m. distance in air, even when the active product is enclosed in a glass vessel. ”

“ RADIATION RESISTANCE—The resistance, which if inserted in place of an antenna, would consume the same amount of power as that radiated by the antenna.
RADIO FREQUENCIES—Electromagnetic frequencies that fall between 3 kilohertz and 300 gigahertz and are used for radio communications.
RADIO HORIZON—The boundary beyond the natural horizon in which radio waves cannot be propagated over the earth’s surface.
RADIO WAVE— (1) A form of radiant energy that can neither be seen nor felt; (2) An electromagnetic wave generated by a transmitter. ”

“ An apparatus of this kind is very convenient for testing the absorption of the radiations by solid screens, as well as for making comparative studies of the activity of different bodies. Unless very active preparations of radium are employed, a battery of 300 volts is sufficient to ensure saturation when the plates are not more than 5 centimetres apart. If substances which give off a radio-active emanation are being tested, the effect of the emanation can be eliminated by passing a steady current of air from a gas bag between the plates. ”

“ If the active matter in the atmosphere consists mainly of the radium emanation, the active deposit on a negatively charged wire, exposed in the open air, should initially consist of radium A, B and C. The curve of decay should be identical with the decay curve of the excited activity of radium, measured by the α rays, that is, there should be a rapid initial drop corresponding to the initial 3 minute change, then a slow rate of variation, the activity after several hours decaying to half value in about 28 minutes (see section 222) . ”

“ Radio-active bodies may be employed in the study of atmospheric electricity. The active substance is enclosed in a little box of thin aluminium fixed at the extremity of a metal wire connected with the electrometer. The air is made to conduct in the neighbourhood of the end of the wire, and the latter adopts the potential of the surrounding air. Radium thus replaces, with advantage, the flames or the apparatus of running water of Lord Kelvin, till now in general use for the investigation of atmospheric electricity. ”

“ I devised a very satisfactory method for determining the quantity of radium by the means of a radioactive gas produced by it and called "emanation." This method, frequently used in my laboratory, permits of the measurement of very small quantities of radium (less than a thousandth of a milligramme) , with a fair precision. More important quantities are often measured by their penetrating radiation, named Gamma-rays. For this we also possess in my laboratory a suitable equipment. It is easier and more satisfactory to measure the radium by the emitted rays, than to weigh it in a balance. ”

“ This concentration of energy provides a maximum amount of energy to be reflected off of a target, making detection of the target much more probable.
How does the parabolic reflector focus the radio waves? Radio waves behave much as light waves do. Microwaves travel in straight lines as do light rays. They may be focused or reflected, just as light rays may be. In figure 2-40, a point-radiation source is placed at the focal point F. The field leaves this antenna with a spherical wavefront. ”

“ The conservation of radio-activity applies not only to the radiations taken as a whole, but also to each specific type of radiation. If the emanation is removed from a radium compound, the amount of β radiation of the radium at once commences to decrease, but this is compensated by the appearance of β rays in the radiations from the vessel in which the separated emanation is stored. At any time the sum total of the β radiations from the radium and the emanation vessel is always the same as that from the radium compound before the emanation was removed. ”

“ The rate of movement is most simply obtained by observing with a stop-watch the time taken for the spot of light, after the motion has become steady, to pass over 100 divisions of the scale. As soon as the observation is made, the plate B is again connected with earth, and the electrometer needle returns to its original position.
In most experiments on radio-activity, only comparative measurements of saturation currents are required. ”

“ FIG. 63.—Complete receiving set, consisting of two "Perikon" detectors, potentiometer, loose coupler, variable condenser, etc.
There are probably more different forms of detector than any other piece of radiotelegraph apparatus. Those in most common use to-day are the mineral detectors. A small crystal of certain minerals, iron pyrites, silicon, galena, etc., is placed between two contact points which are adjustable so that the pressure may be regulated and the most sensitive portion of the mineral selected. A telephone receiver is shunted across the terminals of the detector. ”

“ Since a body which is radio-active must ipso facto be undergoing change, it follows that none of the active products, for example, the emanations and Th X, can consist of any known kind of matter; for there is no evidence to show that inactive matter can be made radio-active, or that two forms of the same element can exist, one radio-active and the other not. For example, half of the matter constituting the radium emanation has undergone change after an interval of four days. ”

“ In making investigations of strongly radio-active bodies, particular precautions must be observed for obtaining delicate determinations. The different objects used in the chemical laboratory and those used for physical experiments soon acquire radio-activity, and act upon photographic plates through black paper. Dust particles, the air of the room, clothing, all become radio-active. The air of the room becomes a conductor. In our laboratory the evil has become acute, and we no longer have any apparatus properly insulated. ”