traced and their properties studied that, as the reader will see, we know far more about the ion than we do about the uncharged molecule. With the discovery and study of Cathode rays, Röntgen rays and Radio-activity a new era has begun in Physics, in which the electrical properties of gases have played and will play a most important part; the bearing of these discoveries on the problems of the Constitution of Matter and the Nature of Electricity is in most intimate connection with the view we take of the processes which go on when electricity passes through a gas. I have endeavoured to show that the view taken in this volume is supported by a large amount of direct evidence and that it affords a direct and simple explanation of the electrical properties of gases. The pressure of my other duties has caused this book to be a considerable time in passing through the press, and some important investigations have been published since the sheets relating to the subjects investigated were struck off. I have given a short account of these in a few Supplementary Notes. My thanks are due to Mr C. T. R. Wilson, F.R.S., for the assistance he has given me by reading the proofs and I am indebted to Mr Hayles of the Cavendish Laboratory for the preparation of the diagrams. CAVENDISH LABORATORY, CAmbridge. August, 1903. J. J. THOMSON. I PREFACE TO THE SECOND EDITION. HAVE made many additions to this edition and a considerable part of it has been rewritten, in the hope of introducing new material in a more logical and connected form than by merely adding new paragraphs to the old edition. This has increased the size of the book; on the other hand the publication, since the first edition of this book, of Rutherford's Radioactivity has enabled me to omit some matter fully treated by Rutherford. So many researches on Discharge through Gases have been made since the issue of the first edition that anything like a complete account of them is impossible within the space at my disposal. I have therefore limited myself to those which seemed most capable of testing the accuracy of the view of Electric Discharge advocated in this book. The light which can be thrown by the study of the Electrical Phenomena occurring in Gases on many of the most interesting questions in Physics is now generally recognised, and the more the subject is studied the wider are seen to be its applications and the greater the opportunities for further research. I take this opportunity of expressing the gratitude which all students of this subject must feel to the Société de Physique of Paris for the publication of the collection of original papers on Discharge through Gases in the volumes Ions, Électrons, Corpuscles, edited by MM. H. Abraham and P. Langevin. J. J. THOMSON. CAVENDISH LABORATORY, CAMBRIDGE. September, 1906. TABLE OF CONTENTS. Properties of a Gas when in the conducting state 9 84 CHAPTER I. ELECTRICAL CONDUCTIVITY OF GASES IN A NORMAL STATE. 1. A GAS in the normal state conducts electricity to a slight, but only to a very slight, extent, however small the electric force acting on the gas may be. So small however is the conductivity of a gas when in this state, and so difficult is it to eliminate spurious effects, that there have been several changes of opinion among physicists as to the cause of the leakage of electricity which undoubtedly occurs when a charged body is surrounded by gas. It was thought at first that this leakage took place through the gas; later, as the result of further experiments, it was attributed to defective insulation of the rods or threads used to support the body, and to the dust present in the gas; quite recently however it has been shown that there is a true leak through the gas which is not due to the dust or moisture the gas may happen to contain. 2. The escape of electricity from an insulated charged body has attracted the attention of many physicists. Coulomb*, whose experiments were published in 1785, came to the conclusion from his investigations on the loss of electricity from a charged body suspended by insulating strings, that after allowing for the leakage along the strings there was a balance over, which he attributed to a leakage through the air. He explained this leakage by supposing that the molecules of air when they come into contact with a charged body receive a charge of electricity of the same sign as that on the body and are then repelled from it, carrying off some of its charge. We shall see later on that this explanation is not tenable. * Coulomb, Mémoires de l'Académie des Sciences, 1785, p. 612. T. G. 1 Matteucci* experimenting on the same subject in 1850 also came to the conclusion that there was a leakage of electricity through the gas; he was the first to prove that the rate at which this leak takes place is less when the pressure of the gas is low than when it is high. He found also that the rate of leak was the same in air, carbonic acid and hydrogen. On the other hand Warburg found that the rate of leak through hydrogen was only about half of that through air and carbonic acid; he agreed with Matteucci with regard to the equality of the rate of leak through these gases and could detect no difference between the leaks. through dry and moist air; he confirmed Matteucci's observations on the effect of pressure on the rate of leak. Warburg seemed inclined to suspect that the leak was due to dust in the gases. The belief in dust being the carrier of the electricity was strengthened by an experiment made by Hittorf in which a small carefully insulated gold-leaf electroscope was placed in a glass vessel filled with filtered gas; the electroscope was found to have retained a charge even after the lapse of four days. We know now from recent experiments that the smallness of the leak observed in this case was due to the smallness of the vessel in which the charged body was placed rather than to the absence of dust. Further experiments on this subject were made by Nahrwold§ and by Narr who showed that the rate of leak from a charged hollow sphere was not increased when the temperature of the sphere was raised by filling it with hot water. Boys¶ made an experiment which showed very clearly that, whatever the cause of the leak might be, it was not wholly due to want of insulation in the supports of the charged body; in this experiment he attached the gold leaves of an electroscope first to a short and thick quartz rod and then to a long and thin one, and found that the rate of leak of electricity from the gold leaves was the same in the two cases; if the leak had been along the supports it would have * Matteucci, Annales de Chimie et de Physique, xxviii. p. 390, 1850. + Warburg, Pogg. Ann. cxlv. p. 578, 1872. Hittorf, Wied. Ann. vii. p. 595, 1879. § Nahrwold, Wied. Ann. v. p. 460, 1878; xxxi. p. 448, 1887. Narr, Wied. Ann. v. p. 145, 1878; viii. p. 266, 1879; xi. p. 155, 1880; xvi. p. 558, 1882; xxii. p. 550, 1884; xliv. p. 133, 1892. ¶ Boys, Phil. Mag. xxviii. p. 14, 1889. |
