as are found in a condenser, to use C.G.S. electrostatic units; but in the great majority of cases either the C. G. S. electromagnetic or the practical system, which is a modification of the electromagnetic system, is used. 519. The C. G. S. electrostatic system. This system is based on the attraction between two stationary charges, or electrostatic forces. The fundamental unit is the unit of charge, or quantity. It is defined from equation (8) of section 390, F-9192, d2 which gives the force in dynes between two small charges d centimeters apart in a vacuum.* From this unit and the C.G.S. units of mechanics, the other electrostatic units are derived by the aid of the fundamental equations. For example, a unit difference of potential is a difference of potential such that the work done in carrying an electrostatic unit of charge between the two places is equal to 1 erg. 520. The C. G. S. electromagnetic system. This system is based on a magnetic quantity-the unit pole. The first electrical term to be defined is current, which is defined in terms of the force, in dynes, that it will exert on a unit pole. The definition of current is thus based on the fact that a current produces a magnetic field; hence the name electromagnetic. The other units are all defined in a logical way by means of the fundamental equations of electricity and those of mechanics. This will be outlined for the more commonly used quantities. The student should note that the first is defined from its mechanical action, by a force measured in dynes. The other quantities are all defined in terms of those which precede. There is no "reasoning in a circle." Unit pole: based on the force one pole exerts on another. The difference in the force between the charges when in a vacuum or in the air is usually negligible. 521. The numerical relation between electromagnetic and electrostatic units. It is found that there is in each case a remarkable relation between the corresponding units in the electromagnetic and electrostatic systems. The ratio between any two is equal to the numerical value of the speed of light (3 × 1010 centimeters per second) or in some cases to the square of it. The following table gives the values of this relation: Current: 1 electromagnetic unit = 3 × 1010 electrostatic units. Difference of potential: 1 electromagnetic unit 1 3 × 1010 electrostatic unit. Resistance: 1 electromagnetic unit = 1/(3 × 1010)2 electrostatic unit. 522. The practical system. The practical system is based on the electromagnetic, and differs from it only in that units of more convenient size are used. But in making this change two things were kept in mind: first, the change was always made by some factor of 10 (that is, by merely moving the decimal point); second, the changes were so made that the fundamental equations would still hold. For example, Ohm's law is A unit called the volt, 108 times as large as the electromagnetic C.G.S. unit was taken; the unit thus obtained was nearly the same as the electromotive force of the Daniell cell, which had become a sort of standard. Similarly, by taking a unit called the ohm, 10o times as large as the electromagnetic unit, a unit was obtained nearly equal to the resistance of a column of mercury 1 square millimeter in section and 100 centimeters long, which had been used as a standard of resistance. Thus two units, the volt and the ohm, were nearly equal to those which were already in use. In order that Ohm's law should still be true it became necessary to take a new unit of current, the ampere, which would be one tenth of the electromagnetic unit. This may be verified; for if I = V/R, then *A cell of the same chemical structure as a gravity cell. The following table gives the relations between the practical and the electromagnetic units. Since the practical units were defined in terms of the electromagnetic units, this table really gives the definition of the practical units. Current: 1 ampere = 1/10 C.G.S. electromagnetic unit. units. 523. International units. Since the units of the electrostatic and electromagnetic systems are derived units, it was necessary to find the fundamental quantities by experiment. The values of the ampere, ohm, volt, etc. were all determined experimentally. In the early days the experimental methods used in finding these quantities were not accurate as compared with the work that can now be performed in our best laboratories. As experimental methods were improved, the values of electrical units became known more and more accurately. At several different times the most recent experimental values were adopted as standard. For obvious reasons there soon arose a demand for fixed standards that would not have to be changed from time to time. Hence, in 1908, an International Electrical Conference decided to take the best-known values of the electrical standards and make fixed standards of them. It was recognized that these might be different from the true values, and that subsequent experiments might reveal this; however, it was decided, in the interests of stability, to ignore this, especially as it was certain that the deviations would be very small. These units were named the international units. The principal international units are defined as follows: 1. The international ampere is the unvarying electric current/which, when passed through a solution of silver nitrate in water (made in accordance with certain specifications), deposits silver at the rate of 0.00111800 gram per second. 2. The international ohm is the resistance offered to an unvarying electric current by a column of mercury at the temperature of melting ice, 14.4521 grams in mass, of a constant cross-sectional area, and of a length of 106.300 centimeters. 3. The international volt is the electrical pressure which, when steadily applied to a conductor the resistance of which is 1 international ohm, will produce a current of 1 international ampere. For practical purposes the working standard for measuring voltage is the Weston cell, the electromotive force of which at 20° C. is 1.0183 international volts. Theoretically there are two different amperes: the international, and the true, which is defined in terms of the C. G. S. electromagnetic unit. The same classification holds for the other practical units. However, the numerical difference between the two amperes, or the two ohms etc., is very small,-probably not greater than 1 or 2 parts in 10,000. } CHAPTER XXXVII CONDUCTION OF ELECTRICITY THROUGH GASES Conduction through gases at ordinary pressures, 524. Ionization by radiation, 525. Ionization by collision, 526. Spark discharge, 527. Discharge at reduced pressures, 528. Geissler tubes, 529. Cathode rays, 530. Nature of cathode rays, 531. X-rays, 532. X-ray tubes, 533. Nature of X-rays, 534. Electronic emissior used in the rectification of alternating currents, 535. Electronic emission used in amplifiers, 536. Emission of electrons, 537. Millikan's method for measurement of the fundamental charge of electricity, 538. 524. Conduction through gases at ordinary pressures. The fact that a good electroscope will retain its charge for a long time shows that the air around it is a very poor conductor. All gases at ordinary temperatures and pressures are usually good insulators. But there are circumstances under which air becomes conducting; and when it does, it seems to behave in a manner somewhat similar to that of an electrolyte. It will be remembered that an electrolyte is a conductor because it is ionized; that is, it contains charged ions which are free to move. To make a gas a conductor some of its molecules or atoms must be separated into oppositely charged parts, or ions. These free ions may be electrons, or charged atoms, or charged clusters of atoms. Chemical changes may leave a gas temporarily ionized; for example, the vapors arising from a flame are usually ionized for a short time, often less than a second. When a candle flame is held underneath a charged conductor, the conductor will be quickly discharged; or the candle flame may be held somewhere near and the rising vapors blown against the conductor. It is very easy to show in this way that vapors may conduct electricity. Other methods by which gases at ordinary pressures may be made conducting will be discussed later. 525. Ionization by radiation. There are several forms of radiation that will ionize any gas they pass through. The short light |