current- would flow in the circuit PBF. A little consideration will show that the grid may produce the following different effects:

1. If the grid is charged negatively, the charge will repel the electrons from F and tend to prevent them from reaching the plate P, thus decreasing the current in the circuit comprised of the plate, the battery B, and the filament.

2. If the grid is given a small positive charge by some outside agency, the attractive force this charge exerts on the electrons will help the charge on P in moving electrons away from the filament,

thus increasing the current in the plate circuit.

3. Since the grid is near the filament, a small change in its charge will produce a relatively large change in the plate current.

F

P

.....

B

FIG. 336

+

circuit is increased, but the energy for this increased current is obtained from the battery B. This gives the necessary condition for the tube to act as a relay, or amplifier. This will be clearer to the student in the application which follows.

In long-distance telephony, vacuum-tube amplifiers, or relays, are used to increase the energy in the telephonic currents. Fig. 337 shows, in a simplified manner, the connections.*

The weak telephonic current, which has come from a great distance, flows through the primary of the transformer T1. One end of the secondary of this is connected to the grid, the other to the filament. As in Fig. 336, the battery A supplies the current for heating the filament, and the battery B is used to supply the cur

* The connections shown here are obviously incomplete, for this method would permit "talk" to be transmitted in only one direction.

1

rent in the plate circuit. The telephone current in the primary of the transformer T1 causes fluctuations in the potential of the grid, and these, in turn, cause relatively large changes in the current of the plate circuit. The primary of the transformer T2 is connected in the plate circuit. When the current in this circuit fluctuates, induced currents are produced in the secondary of T2. This current is the amplified telephone current. It has been found practicable to produce this amplification without any appreciable distortion* of the sound heard at the receiving end. Moreover, it has also been found practicable to deliver the current from the secondary of T2 to another tube circuit, where it is again amplified.

2

FIG. 337. A thermionic-amplifier circuit

In actual practice a number of amplifiers can be connected in series. The discovery of this vacuum-tube amplifier was one of the important factors in making transcontinental wire telephony possible.

Vacuum-tube amplifiers are used to enable very large audiences to hear a speaker. By using a large number of them the sound may be amplified a million times. It is now possible for a speaker to address a large audience a thousand miles away. He delivers his speech into a transmitter of a long-distance telephone line, and at the other end the electric currents are amplified and delivered to a number of loud-speaking instruments.

537. Emission of electrons. The ionization of air or other gas is so often due to the impact, or collision, of electrons with gas molecules that it is well to review here the principal ways in which free electrons can be produced.

1. Ultraviolet light. When waves of light shorter than those producing the sensation of violet (ultraviolet waves) fall on any substance, they may cause that substance to emit electrons. It is most marked in the case of the electropositive metals, such as zinc, sodium, and potassium. If a piece of polished zinc is connected to an electroscope and charged with negative electricity, the electroscope shows that the charge is rapidly lost when the zinc is exposed to light from a naked carbon arc, which is rich in ultraviolet light. This is due to the emission by the plate of negatively charged electrons.* But if the zinc is charged positively, it will not lose its charge when illuminated. This is because the positive charge on the zinc, attracting negatively charged electrons, prevents them from leaving the zinc. No ions are produced which can carry away a positive charge.

If ultraviolet light falls on the terminals of a spark gap, a spark can be produced by a lower potential than when no light is used. Why? It is necessary to illuminate only the negative terminal.

The effect produced by ultraviolet light is often called the photoelectric effect.

2. X-rays. The atoms of all substances-whether solid, liquid, or gaseous-emit electrons when they are illuminated by X-rays. The ionization of gases produced by X-rays is an indirect effect. X-rays cause the emission of electrons at high speeds from molecules of the gas, and these electrons ionize the gas by collision with other molecules.

3. Radiations from radioactive substances. The radiations from radium and other radioactive substances cause electrons to be emitted from atoms which these radiations strike.

4. High temperature. The emission of electrons from hot bodies has been referred to before, in sections 533 and 535. This is an important method and has many useful applications.

538. Millikan's method for measurement of the fundamental charge of electricity. It has been stated before that Millikan showed that small charges are an exact multiple of a certain quantity, the smallest charge of

* Glass is opaque to ultraviolet light; hence a sheet of glass held between the arc and the negatively charged zinc plate will stop the emission of electrons and thus stop the loss of charge.

electricity which apparently exists. This we have called the natural unit (sect. 391). According to Millikan no particle or ion can have a charge which is a fraction of this unit: it must be one or two or three or four etc. of these units. The charge on an electron is always one of these units.

It is worth while to give a simple explanation of Millikan's method of measuring small charges. A tiny drop from a spray produced by an atomizer was permitted to fall through a small hole into the space between two horizontal plates. Such a droplet usually picks up a charge by colliding with an ion in the air (ions can easily be produced in the air by the radiation from radium). The droplet was watched by means of a low-power microscope, and its rate of fall carefully measured. From the rate at which the drop fell through air under the action of gravity alone, its size could be determined (sect. 110), and, the density of the material of the droplet being known, its mass could be computed. Then the two parallel plates were charged with electricity so as to drive the droplet up or down, as desired. From the velocity produced by the charged plates the electrical force on the droplet could be computed. The electric force acting was the strength of the electric field times the charge on the droplet (see equation (9), sect. 392). Since the strength of the field was known, the charge of the droplet could be calculated. The actual details, as may be expected, were much more complicated than this brief description indicates.

In several thousand experiments Millikan found that the charge on these droplets was always a whole number of these unit charges where the unit charge was 4.774 × 10-10 electrostatic units, or 1.591 × 10-19 coulombs. We now believe that all charges, whether large or small, are multiples of this unit.

One result of Millikan's determination of the value of e, the unit charge, is that it gives us (as explained in section 455) an accurate method of determining the masses of atoms. Similarly, if the value of e/m for the electrons in the cathode-ray stream can be determined, the mass of the electrons can be calculated. (The ratio of e/m for an electron can be determined by measuring the deflection of the rays by both a magnetic and an electric field. The theory of this is too involved to be given here.) This is the method by which it was found that the mass of the electron is about one eighteen-hundredth of the mass of the smallest known atom-that of hydrogen.

CHAPTER XXXVIII

RADIOACTIVITY

Introduction, 539. Types of radiation emitted, 540. Effects of electric and magnetic fields, 541. Atomic disintegration, 542. Radioactive substances in the uranium-radium series, 543. Source of the gamma radiation, 544. Structure of an atom, 545. Atomic numbers, 546. Isotopes, 547. Other radioactive series, 548.

539. Introduction. The discovery of X-rays attracted a great deal of attention, not only among scientists but also among the general public. It was apparently an entirely new type of radiation, and hence of high theoretical importance. Moreover, the great practical benefit from the use of this radiation was immediately recognized. Naturally there was much speculation as to just what there was in the vacuum tube that caused this radiation. Since the glass walls of all vacuum tubes which emitted X-rays exhibited visible phosphorescence, it probably occurred to many that the X-ray radiation was in some way associated with this phosphorescence. Becquerel examined minerals and other substances which phosphoresced under the action of light, hoping that they too might give an invisible radiation. In this way he found, in 1896, that uranium and its compounds gave an invisible radiation capable of affecting a photographic plate through black paper or through sheets of aluminum. Becquerel soon found that this new radiation had nothing to do with the phosphorescence: that it could be obtained from nonphosphorescent uranium salts. He found also that these radiations, like X-rays, made air, or any gas which they traversed, electrically conducting.

Soon after Becquerel's discoveries Mme Curie and G. C. Schmidt independently found that thorium and its compounds also emitted the same sort of radiation as that given out by uranium. A little later Mme Curie found in pitchblende some traces of a new element, which she called radium. Radium has since been found in