Electroscope

“ Connect the end of a long wire with the tin plate of the electroscope; coil the other end round your glass tube. Rub the tube briskly, carrying the friction close to the coiled wire. A single stroke of your rubber, if skillfully given, will cause the leaves to diverge. The electricity has obviously passed through the wire to the electroscope. ”

“ Fig. 10.—Electric windmill which operates by the reaction due to the escape of the electric charge from the points.
Electroscopes.—These are instruments for detecting whether a body be electrified or not, and indicating also whether the electrification be positive or negative. The earliest electroscope devised consisted of a stiff straw balanced lightly upon a sharp point; a thin strip of brass or wood, or even a goose quill, balanced upon a sewing needle will serve equally well. Another form of electroscope is the pith ball pendulum, shown in figs. ”

“ When the electroscope is placed on a lead plate 3 or 4 mms. thick, the ionization in the electroscope, due to a radio-active body placed under the lead, is due entirely to the very penetrating rays, since the other two types of rays are completely absorbed in the lead plate. If a circular opening is cut in the base of the electroscope and covered with thin aluminium of sufficient thickness to absorb the α rays, measurements of the intensity of the β rays from an active substance placed under it, can be made with ease and certainty. ”

“ When employing a Volta condensing electroscope, the following is the method of procedure:—The top of the electroscope consists of a flat, smooth plate of lacquered brass on which another plate of brass rests, separated from it by three minute fragments of glass or shellac, or a film of shellac varnish. If the electrified body is touched against the upper plate whilst at the same time the lower plate is put to earth, the condenser formed of the two plates and the film of air or varnish becomes charged with positive electricity on the one plate and negative on the other. ”

“ An electrometer is an instrument by means of which electrical charges or electrical potentials may be measured. Instruments by means of which the existence of electric charges or of differences of potential may be indicated, but which are not capable of affording numerical measures, are called Electroscopes.
An electroscope if sufficiently sensible may be used in electrical measurements, provided we can make the measurement depend on the absence of electrification. ”

“ The passage of the current of electricity is not discoverable by the electroscope, because it is adapted only to indicate electricity of high tension or intensity, such as that produced from the electrical machine, which will pass rapidly through a certain thickness of air, and cause pith balls to stand out and repel each other ”

“ The electroscope being charged to the verge of overflow, the impact of weak electric waves collected by a bit of wire sticking up from the left-hand binding screw precipitates the collapse of the leaves.
Among trigger methods of detecting electric radiation, I have spoken of the Zehnder vacuum tubes; another method is one used by Boltzmann. [10] A pile of several hundred volts is on the verge of charging an electroscope through an air gap just too wide to break down. Very slight electric surgings precipitate the discharge across the gap, and the leaves diverge. ”

“ A metal box has a metal strip B suspended from a block or insulator by means of a bit of sulphur or amber S, and to it is fastened a strip of gold-leaf L. The electroscope is provided with a charging rod C. In a dry atmosphere sulphur or amber is an early perfect insulator, and hence if the air in the interior of the box is kept dry by calcium chloride, the electroscope will hold its charge for a long time. Any divergence or collapse of the gold-leaf can be viewed by a microscope through an aperture in the side of the case. ”

“ In using the instrument the insulated rod to which the gold-leaf is attached is connected to the conductor, the potential of which is being examined. In the use of all these electroscopic instruments it is essential to bear in mind (as first pointed out by Lord Kelvin) that what a gold-leaf electroscope really indicates is the difference of potential between the gold-leaf and the solid walls enclosing the air space in which they move. ”

“ Although the electroscope can be used with advantage in special cases, it is limited in its application. The most generally convenient apparatus for measurement of ionization currents through gases is one of the numerous types of quadrant electrometer. With the help of auxiliary capacities, the electrometer can be used to measure currents with accuracy over a wide range, and can be employed for practically every kind of measurement required in radio-activity. ”

“ Hare is also the inventor of a single gold-leaf electroscope of such great delicacy that it has, he says, enabled him to detect the electricity produced by one contact between a zinc and copper disc, each six inches in diameter (Noad, “Manual,” p. 29; Harris’ “Rudim. Elect.,” p. 50; Silliman’s Journal, Vol. XXXV) . ”

“ Atmospheric Electroscope.—The simplest instrument for ascertaining at any time the electric condition of the atmosphere is an electroscope composed of two equal pieces of gold leaf, suspended from a brass support, and insulated, as well as protected from the movement of the air, by a glass covering. ”

“ When fresh, dry, ground coffee is received on to the disc of the electroscope, as it falls from the mill, powerful electrical excitation is displayed, and this is sometimes so apparent, that the particles cling around the lower part of the mill or to the sides of the cup or basin held to catch it. ”

“ You see before your mind's eye two layers holding each other in bondage. Now, take hold of your loops and lift the glass plate, so as to separate the upper tin-foil from the lower. What would you expect to occur? Freed from the grasp of the lower layer, the electricity of the upper one will diffuse itself over the electroscope so promptly and powerfully that, if you are not careful, you will destroy the instrument by the mutual repulsion of its leaves. ”

“ These researches have established the fact that the air is always in an electrical condition, even when the sky is clear and free from thunder-clouds. The instruments employed for ascertaining the kind and intensity of atmospheric electricity are called Electroscopes. Fig. 65 shows a modification of Saussure’s Electroscope, the basis of which is a narrow-mouthed flint glass bottle with a divided scale to indicate the degree of divergence of the gold leaves or straws. To protect the lower part from rain, it is covered by a metallic shield about five inches in diameter. ”

“ The discovery of the new property of radio-activity in a small number of chemical elements led physicists to ask whether the property might not be found in other elements, though in a much less striking form. Are ordinary materials slightly radio-active? Does the feeble electric conductivity always observed in the air contained within the walls of an electroscope depend on ionizing radiations from the material of the walls themselves? ”

“ Either, separately, is repulsive of its own particles, but attractive of the [Pg 181] other. No electrical excitation can occur without the separation of these two curious states of electricity, and electrical quiescence takes place when the two electricities are brought together; hence the fall of the gold leaves repelled by rubbed wax when the excited glass is brought towards the disc of the electroscope. This experiment may be reversed by repelling the leaves first with the excited glass, and then bringing the rubbed wax, when the same effect takes place. ”

“ You soon satisfy yourself that the electricity passes with greater facility through the wire than through the string. Substitute for the twine a string of silk. No matter how vigorously you rub you can now produce no divergence. The electricity cannot get through the silk at all. [4]
Mr. Cottrell, who has been recently working very hard for you and me, has devised an electroscope which we shall frequently employ in our lessons, M, Fig. 6, is a little plate of metal, or of wood covered with tin-foil, supported on a rod of glass or of sealing-wax. ”

“ At four hundred-yard intervals are men with electroscopes like this one. My attempt to locate Saranoff by means of wave detectors was a failure. That proved that the ray he was using is not of the wave type. The other common ray is the cathode ray type which does not consist of vibrations but of a stream of electrons, negative particles of electricity, traveling in straight lines of high velocity. He must be knocking loose some of the electrons when he collapses the atoms. The rate of discharge of these electroscopes will give us a clue to the nearness of his device. ”

“ These are described at p. 608 of the eighth volume of the 1855 “Encyclopædia Britannica.”
According to Mr. Wilkinson (“Elements of Galvanism,” etc., London, 1804, Vol. II. p. 344) , Mr. Read was the first to apply the apparatus called the condenser to the electroscope in order that it should evince small intensities of electricity. He says: “The very minute portion of the fluid given out by the single contact of two different metals, does not produce any disturbance of the gold leaves; but when several minute portions are accumulated, a separation of the leaves takes place. ”

“ Charge the electroscope with F C until the carbon is strongly repelled.
(E) Bring the positively charged E C slowly near the carbon, and note the result.
(F) Slowly bring the negatively charged E S near the carbon that has been charged by contact with F C.
211. Discussion. This experiment showed that while the ebonite was negatively charged, the flannel was positively charged. One kind of electrification is never produced without the other. It can also be shown that the two kinds are equal in amount.
[82] ”

“ If now a charge be put in contact with the body of a, the electroscope will show by its leaves whether the rod tested conducts or not. The leaves separate instantly when conducting substances are tested, while no action results with insulators. In testing some materials for conductivity the leaves are found to diverge gradually. Such bodies are said to be poor conductors. All degrees of conductivity are found. The metals are the best conductors. The best insulators are rubber, mica, shellac, glass, silk, porcelain, paraffin, and oils. ”

“ He regarded the outer layers as different from the mass of the metal and having a much greater specific resistance. He supposed that when two pieces of metal are pressed together the molecules diffuse across the surface, modifying the surface layers and increasing their conductivity.
§ 8. Generators of Electric Waves.—Bose (Phil. Mag. 43, p. 55) designed an instrument which generates electric waves with a length of not more than a centimetre or so, and therefore allows their properties to be demonstrated with apparatus of moderate dimensions. ”

“ Simpler types of electrometer of greater sensibility can however be readily constructed to give accurate results. The old type of quadrant electrometer, to be found in every laboratory, can readily be modified to prove a useful and trustworthy instrument. A light needle can be made of thin aluminium, of silvered paper or of a thin plate of mica, covered with gold-leaf to make it conducting. The aluminium wire and mirror attached should be made as light as possible. ”

“ The ends of this filament extend out through the glass so that we may connect a battery to them and pass a current of electricity through the wire. If we do so the wire gets hot.
What do we mean when we say “the wire gets hot?” We mean that it feels hot. It heats the glass bulb and we can feel it. But what do we mean in words of electrons and atoms? To answer this we must start back a little way.
In every bit of matter in our world the atoms and molecules are in very rapid motion. In gases they can move anywhere; and do. That’s why odors travel so fast. ”

“ In all electrometers the moveable part is a body charged with electricity, and its potential is different from that of certain of the fixed parts round it. When, as in Coulomb's method, an insulated body having a certain charge is used, it is the charge which is the direct object of measurement. We may, however, connect the balls of Coulomb's electrometer, by means of fine wires, with different conductors. The charges of the balls will then depend on the values of the potentials of these conductors and on the potential of the case of the instrument. ”

“ If other electrons follow the first within a wire, their passage along the wire will be what is called an electric current.
When the electron is subjected to an acceleration, a transverse wave is produced, and an electromagnetic radiation is generated, of which the character may naturally change with the manner in which the speed varies. If the electron has a sufficiently rapid periodical movement, this wave is a light wave; while if the electron stops suddenly, a kind of pulsation is transmitted through the ether, and thus we obtain Röntgen rays. ”

“ The action of detectors, etc., depends upon this fact; and, strange to say, the magnetic field about the wire disappears the instant the current ceases to pass. The combination, thus, of a coil of wire and a magnetic needle, properly arranged, makes an instrument with which the presence of electricity can be detected. When the strength of a current is to be measured, or the strengths of two currents are to be compared, the apparatus is called a galvanometer. The method of making these pieces of apparatus will depend upon the strength of current to be tested or measured. ”