254 Prof. J. A.. Fleming on the Poulsen Arc as a 



alternator. In the first no condenser was placed in the secon- 

 dary circuit and the primary current was 3*5 amperes. In the 

 second the secondary coil was connected to condenser III. and 

 the current in the primary was 1 ampere. In this case the 

 secondary circuit was nearly in tune with the alternator, and 

 resonance, accompanied by violent sparking across an air-gap 

 in parallel with the condenser, occurred when the machine 

 was running at a certain speed. 



Bangor, May 1907. 



XX. Some Observations on the Poulsen Arc as a means of 

 obtaining Continuous Electrical Oscillations. By J. A. 

 Fleming, M.A., D.Sc, F.R.S., Professor of Electrical 

 Engineering in University College, London *. 



THE interesting discovery of Mr. Poulsen that an electric 

 arc between a carbon and a metal electrode formed in 

 an atmosphere of coal-gas and traversed by a magnetic field, 

 when shunted by a condenser and inductance, as in the 

 Duddell musical arc, furnishes a means of obtaining undamped 

 electrical oscillations, of much higher frequency than with a 

 plain carbon arc in air, has attracted much attention from its 

 technical application in wireless telegraphy. Having made 

 a careful examination of the matter during the last six or 

 seven months, the record of some of my observations in 

 connexion with it may be of interest to other physicists who 

 are studying the arc method of exciting oscillations. The 

 apparatus with which I have worked, constructed on the same 

 lines as that of Mr. Poulsen, consists of a stout brass cylinder 

 14 cms. in diameter and 19 cms. long, which is soldered 

 across another brass box 7 inches by 18 cms. by 25 cms. 

 high, the cylinder being open at both ends (see fig. 1). 

 The interspace between the cylinder and the containing box 

 serves as a water-jacket, and through it cold water is made 

 to flow by inlet and exit pipes. The ends of the cylinder 

 are closed gas-tight by marble plates, and these are pierced 

 by holes in which brass sleeve tubes are fitted. 



The cylinder has also inlet and exit pipes for coal-gas. In 

 addition, a pair of brass tubes pass through the sides of the 

 box and the cylinder, and in these are fitted soft iron rods 

 4 cms. in diameter and 20 cms. long. The inner ends are 

 about 3 to 4 cms. apart. In the chamber so constructed an 

 electric arc is formed between the edge of a stout cylindrical 

 carbon rod 2*5 cms. (1 inch) in diameter, which passes 



* Communicated by the Physical Society : read June 14th, 1907. 



