1909] Electrical Striations. b77 



WEEKLY EVENING MEETING, 



Friday, April 2, 1909. 



Sir William Crookes, LL.D. D.Sc. F.R.S., Honorary 

 Secretary and Vice-President, in tlie Chair. 



Professor Sir J. J. Thomson, M.A. LL.D. D.Sc. F.R.S. 3f.R.L ; 

 Professor of Natural Philosophy, Royal Institution. 



Electrical Striatiojis, 



One of the most conspicuous features of the electric discharge through 

 gases, when the pressure is within certain limits, is the exceedingly 

 well-marked alternations of light and darkness whicli occur in the 

 positive column. These alternations, which are called striations, are 

 so varied and beautiful that since their discovery by Abria in 1843 

 they have attracted the attention of many physicists. Grove, Gassiott, 

 Spottiswoode and Moulton, De la Rue and Mtiller, Crookes, Wood, 

 Skinner, H. A. Wilson, and Willows, have published important re- 

 searches on the conditions under which the striations are produced ; 

 on the influence upon them of such things as the nature and pressure 

 of the gas, the size of the tube, the current passing through it ; and 

 on the distribution of the electric force in the neighbourhood of a 

 striation. The investigations described in the following paper relate 

 for the most part to the last of these questions, and were made with the 

 object of testing a theory of the striations which I gave in my Treat- 

 ise on the Conduction of Electricity through Gases. For these experi- 

 ments I used tubes fitted with Wehnelt cathodes, i.e. the cathode was 

 a strip of platinum-foil heated to redness, and having on it a spot of 

 lime or barium oxide.* 



With these cathodes large currents can be sent through the tube, 

 and remarkably bright and steady striations obtained at lower pres- 

 sm-es and with smaller potential-differences than with the ordinary 

 type of discharge. The pressure has, however, to be low, consider- 

 ably less than 1 mm. of mercury, to get the full advantages from 

 these cathodes. The first point to which attention was directed was 

 the distribution of electric force along the line of the discharge. In- 

 vestigations on this point have already been made by Skinner and 

 H. A. Wilson, but it seemed to me that the steadiness of the striations 



* My assistant, Mr. Everett, has found that these cathodes can be very 

 easily made by letting a drop of sealing-wax fall on the foil, and then burning 

 away the combustible matter by heating the foil to incandescence. Sealing- 

 wax seems to contain large quantities of some salt of barium. 



