Experiments on the dark space in vacuum tubes

1. When an induction spark passes through an exhausted vacuum tube we see, firstly, a luminous layer coating the cathode, next a dark space; beyond the outer edge of this dark space comes a luminous envelope, then another blank, sometimes called “Faraday’s dark space,” and, lastly, the positive column. Between the second dark space and the positive column, if the exhaustion is suitable, stratifications occur. In the present paper I speak of the first dark space extending from the luminous layer on the cathode to a more or less sharply defined luminous boundary. The luminous coating on the cathode is produced by the ionisation of the atoms of residual gas and the union of the electrons from the metal with the positive ions, with liberation of a further jet of electrons starting from the neighbourhood of the cathode with velocities of the order of that of light. 2. The dark space is a measure of the mean free path of the electrons, and its outer luminous margin is the scene of the collisions between free electrons and the column of positive ions. It varies in size with the degree of exhaustion. At a pressure of about 4 mm. it begins to appear as a narrow space, a fraction of a millimetre removed from the negative pole, and grows larger as the exhaustion increases. At a pressure of about 3 mm. the margin of the dark space is about 4 mm. from the negative pole. At an exhaustion of 0·25 mm. it is about the best size for such work as I now describe. When the exhaustion is pushed, further, the outer boundary becomes indistinct and soon fades away, the dark space now filling the tube, the walls of which glow with a phosphorescent light. It is to the phenomena occurring within this dark space that I have devoted years of work, and I now have the honour of presenting to the Society some account of the results of my prolonged investigations; in parts they lead to conclusions which have been already made public by other observers.