RADIO COMMUNICATION
Nature of Electricity.—Everyone is more or less familiar with elemen tary experiments having to do with electrically charged bodies. Fur, if rubbed on a dry day, crackles and gives off minute sparks; a glass rod rubbed with a cloth becomes electrified and will attract small bits of paper, cotton, etc. ; due to wind friction, and other causes, clouds become intensely electrified and are able to break down the insulating strength of the air and produce sparks thousands of feet long. In what way does an electrified body, or electrically charged body, differ from one in the uncharged, or neutral, state? A reasonable answer to this question is found in the modern conception of the constitution of matter.
Electrons.—It has been firmly established that every atom of matter is charged with minute particles 1 of negative electricity, so-called electrons. An electron, when detached from the atom of matter with which it was associated, shows none of the properties of ordinary matter. It does not react chemically with other electrons to produce some new substance; moreover, all electrons are similar, no matter from what type of. atom they have been extracted. Thus an electron from the hydrogen atom acts precisely the same as the electrons from atoms of oxygen, iron, chlorine, or any other substance. It seems that the electron is nothing but electricity. 1 It may seem difficult at first to think of electricity as made up of separate, dis crete, quantities instead of a continuous distribution of electric charge, but it is pointed out that according to modern concept energy ilself is always present as a certain number of unit quantities; that is, energy itself is to be " counted " in terms of the smallest possible quantity, called a " quantum."
It is definite in amount, always being exactly the same, and is generally believed to be the smallest possible quantity of electricity, i.e., electricity cannot be subdivided into quantities smaller than the electron. The constants of the electron are: Radius = 2X10 "cm. ; mass = 8.8 X10-28 grams; charge = 1.59X10" 19 coulomb.1 The mass of the electron depends upon the velocity with which it is moving; the value given here holds good only if the electron is traveling at velocities considerably less than the velocity of light, say less than 109cm./sec. For many years it has been the custom for physicists to speak of positive electricity and negative electricity; from this standpoint the electron is negative electricity. All electrons are the same kind, or polarity, hence it follows that the electron is the smallest possible quantity of negative electricity.
The structure of the atom itself, whatever it Fia. 1. — Conventional may be, is always charged electrically positive; in model of a simple, the normal atom there are enough electrons to just neutral, atom. neutralize the positive charge of the atom itself. The normal atom acts like an uncharged body, therefore, not because it has no electrical charge associated with it, but because it has just as much negative charge as it has positive charge, and these two charges neutralize one another in so far as action of the atom on other bodies is concerned. If one electron is removed from the atom by some means or other (represented in Fig. 2) the balance between positive and negative charge is destroyed; an excess of positive charge exists on the atom and the atom is positively charged. The electron which has been removed from the atom constitutes a negative charge. If the electron is allowed to go back 1 The student who is particularly interested in the theoretical and experimental work from which these values are obtained is referred to " Conduction of Electricity through Gases," by J. J. Thomson. s In recent years much work has been done in investigation of the structure of the atom; an interesting and elementary exposition of some of the modern views is given in " The Nature of Matter and Electricity," by Comstock and Troland.
Number of Electrons Removable from an Atom.—Although there may be a great number of electrons associated with an atom or molecule it is generally not possible to remove more than one; in a body which is positively charged most of the atoms are neutral, having their proper complement of electrons; others have had one electron removed. If but few of the atoms of a body have had an electron removed the body has a small charge; the more highly the body is charged the more deficient atoms there are on it. From this viewpoint it seems that the Fl0 2.—Conventional model of a amount of charge on a body should be simple atom charged positively, counted; the charge consists of discrete one of its electrons being free, things. Instead of saying that a body has a certain amount of negative electricity on it, we might more reason ably say that a certain number of electrons have been deposited on it. Electric Fields.
If a light substance, such as a pith ball, is touched to a charged body, it becomes charged with electricity of the same polarity as that on the body itself ; as like charges repel one another the pith ball will be repelled from the charged body. By experimenting it may be found that the repulsive force between the pith ball and the original charge exists even when there is considerable distance between the two. The Bpace surrounding a charged body is evidently under some kind of strain which enables it to act upon a charged body with a force, attractive or repulsive, according to the relative polarities of the two charges. This space surrounding a charged body, in which another charged body is acted upon by a force tending to move it, constitutes an electric field, sometimes called an electrostatic field.
Such an electric field surrounds every charged body; it really extends to infinity in all directions from the charged body, but as the force becomes very small as the distance is increased it is generally considered that the electric field due to a charge extends but a short distance from the charge. For example, the field due to a piece of charged sealing wax is negligible at a point a few feet distant from the wax, so we say that the field of this charge extends but a few feet from the wax. On the other hand the electric field produced by a large, highly charged, wireless antenna may extend several thousand feet from the antenna.
er, the electrons in a metallic conductor are more or less free to pass from one atom of the substance to another; they are continually moving around the complex molecular structure of atoms comprising the metal. When the rod of Fig. 8 is brought into the neighborhood of the charged ball the electric field due to the charge on the ball acts on the free elec trons of the rod, attracting them. Hence the free electrons of the rod tend to congregate at that end of the rod which is nearest to the ball; they constitute the negative charge at this end of the rod.
But if the rod was uncharged before coming into the influence of thf charged ball there must be just enough electrons on it to neutralize the positive charges of the atoms. If more than a proper portion of the elec trons gather at one end of the rod there must necessarily be a shortage of them at the other end. This shortage of electrons at the end C of the rod constitutes the positive charge at this end.
An Essential Difference between Positive and Negative Charge.—As before stated, the electrons from all substances are the same; the elec trons have none of these qualities by which we distinguish and classify matter. It is possible to have electrons in space entirely devoid of matter; a negative charge can exist in a perfect vacuum. The question may be raised—How can it be a perfect vacuum if there are electrons present? By a vacuum we mean a space in which there is no material substance, solids which can be bodily removed, liquids which can be poured out, or gases which can be pumped out. A glass vessel which has been evacuated as perfectly as modern pumping methods can accomplish may nevertheless be filled with millions of electrons.
From our conception of the positive charge, however, it is evident that a positive charge must always be associated with matter, in fact the smallest positive charge is an atom of matter from which an electron has been removed. If in a glass bulb supposedly evacuated it can be shown that under some circumstances positive charges exist the vacuum is only partial; to the same extent that positive charges occur in the supposedly vacuous space, must matter of some kind (generally gas) be present.
The Electric Current.—The electric current is more familiar to every one than the electric charge. The current manifests itself in various ways, by generating heat and light, by producing mechanical forces such as those required to ring a doorbell or pull a subway train, by producing chemical changes such as occur in the production of aluminum, or electro plating, by producing death if it flows through a living organism with sufficient intensity, etc. Older conceptions of the electric current made it a peculiar fluid of some kind, others made it consist of two fluids with different properties.
From the electron standpoint the conception of the electric current is easy to comprehend and enables one to give a fairly logical explanation of the various actions of the current. Nature of the Electric Current. An Electron in Motion Constitutes an Electric Current.—The amount of electricity on one electron is so small that the current produced by one electron in motion would not be detect able by the finest current-measuring instrument, even the most sensitive. To produce currents of the magnitude occurring in every-day experi ence requires the motion of electrons measured in billions of billions per second.
Although the progressive motion of the electrons is very slow, as indicated above, it must not be thought that the actual velocity of the electrons is small. If we assume the " equi-partition of energy " idea of thermc-dynamics and thus calculate the average velocity of the electrons in a copper wire, at ordinary temperature, we obtain a result of about 6X106 cm. per second. That is, even when no current is flowing in the wire the electrons have a haphazard motion, due to the thermal agita tion of the atoms (or molecules), which give them, on the average, a velocity of about 35 miles per second. Now when current flows the required progressive velocity of the electrons is only a fraction of a centimeter per second; with a current so large that the copper wire is heated to the melting-point the velocity of drift of the electrons is less than 1 cm. per second.
Thus an accurate concept of the electric current in a conductor shows it to be an inappreciable " drift " of the electrons which have, due to temperature effects, hetero geneous velocities millions of times as great as the velocity of drift. The reason for the slow progressive motion of the electrons is to be seen in the tremendous number of collisions they have with the molecules of the substance. A given electron, acted upon by the potential gradient in the wire carrying current, accelerates very rapidly and would acquire tremendous velocities if it did not continually collide with the more massive molecules; the mean free path of the free electrons in a copper wire is so small that, between successive collisions, the electron falls through a very small potential difference and hence gains a velocity (along the con ductor) due to the current, which is extremely small.
Difference between Conductors and Insulators from the Electron Viewpoint.—When a conductor is carrying an electric current the elec trons throughout the substance of the conductor are moving gradually along through the substance of the conductor. Now in a solid body, such as a metallic conductor, the atoms or molecules comprising the sub stance are practically fixed in position. They are not actually stationary in space at ordinary temperature of course; as a matter of fact the atoms have an irregular to-and-fro motion similar to that of the electron. But there cannot be a progressive motion of the atoms as there may be of the elec trons. The reason for this is more or less evident. Suppose a copper wire is fastened to the terminals of a battery and that current is flowing as indicated in Fig. 10. The electrons move all the way around the circuit through the wire, connections, solution in the battery, etc.
As the atoms of copper are charged positively after an electron has left them it might seem that as the electrons move from B to A through the wire the atoms would move from A to B, then into and through the battery and so back to the wire. But the atoms are the real substance of the wire, and hence if the atoms should progress one way or the other it would result in the copper itself being carried from one end of the wire to the other and then through the battery. This state of affairs is not possible in solid bodies like metals, it would result in the mixing of metals wherever a current left one metal and went into another.
In chemical solutions, e.g., copper sulphate in water, the salt mole cule breaks up into two parts, one of which has one electron more than its proper number, the other part lacking one electron. The two parts of the molecule are called ions; the metallic ion (in above case, copper) lacks one electron and so is charged positively. If now a current is passed through such a solution the metallic ion does move through the solution and is carried from the solution to one of the wires by which the current is lead into the solution. Here the copper itself is transported by the current and we have the process of electro-plating.