194 Scientific Intelligence. 



trode. Spottiswoode and Moulton found that the time which the 

 positive electricity needed to traverse the tube was shorter than 

 that which the negative electricity required to leave the cathode. 

 They believed that the time occupied by a discharge to traverse 

 a metallic conductor was much shorter than the time required 

 through a discharge tube. J. J. Thomson took a discharge tube 

 15 m long, blackened it except at two points ; threw an image of 

 these two points on a cylinder. When the cylinder was at rest, 

 the points or slits were directly over each other ; when the cylin- 

 der was set in rapid revolution the images of the points separated. 

 When the current was reversed, the images exchanged posi- 

 tion. He concluded that the discharge was independent of the 

 nature of the electrodes and always proceeded from the anode to 

 the cathode, and that for a pressure of 0'8 mm the velocity of the 

 discharge was approximately half that of light. E. Wiedemann 

 and G. C. Schmidt have shown that under certain conditions an 

 apparent velocity of 200 m per second can be observed. Wtillner, 

 Khigi and others have noticed an extraordinarily slow velocity. 

 On account of the divergence of results J. James has studied the 

 question of the velocity in discharge tubes by means of the Abra- 

 ham-Lemoine method of measuring small intervals of time. This 

 method consists in employing changes in polarization of light in 

 the magnetic field, or by electric stress. By these means a time 

 interval of one hundred millionth of a second can be detected. 

 James gives an analytical discussion of his employment of the 

 method which he used to study the discharge of the electric 

 spark in air, and also in a discharge tube. He found results 

 which did not agree with those obtained by J. J. Thomson. 

 They did agree, however, with those of Spottiswoode and Moulton. 

 Further investigations are needed to determine whether from a 

 study of the kind of illumination one can draw conclusions in 

 regard to velocity. — Ann. der JPhys., No. 15, 1904, pp. 954-987. 



J. T. 



8. Extinction of the Electric Spark. — Wireless telegraphy and 

 the study of N-rays give a great interest to the study of the elec- 

 tric spark. John Koch discusses conditions of residual charges 

 of condensers and the fall of potential and other conditions which 

 accompany or cause extinction. He concludes that the causes 

 which lead to this extinction are purely electrodynamic and are 

 identical with those connected with the dying out of the voltaic 

 arc. The author concludes that Heydweiller's contention, that 

 the development of energy is proportional to the time of dis- 

 charge and independent of the current strength, is true only to a 

 first approximation, when very great resistances are in the cir- 

 cuit.— Ann. der Phys., No. 15, 1904, pp. 866-905. j. t. 



9. Exhaustion of Geissler Tubes by the Electric Current. — It 

 has often been noticed that the degree of exhaustion in Geissler 

 tubes, and notably in X-ray tubes, is changed by long continuance 

 of electrical discharges. At first the heat of the discharge increases 

 the pressure and then there comes a lowering of the pressure 



