Section III., 1899. L 13 ] Trans. R. S. C. 



III. — Damping of Electrical Oscillations. 

 By Harriet Brooks, of McGrill University. 

 (Presented by Mr. Deville, and read May 26th, 1899.) 



The oscillatory character of the discharge of a Ley den jar was sus- 

 pected by Henry, in 1842, from observations on the anomalous mag- 

 netization of steel needles placed inside a coil in the discharge circuit. 



In 1857 Feddessen detected the oscillations by putting an air-break 

 in the circuit connecting the inner and outer coatings of the jar. He 

 examined the appearance of the air-break, while the discharge was 

 passing, by means of a rotating mirror, and found that the image con- 

 sisted of a series of bright and dark bands rapidly decreasing in breadth 

 and intensity. When a large resistance was placed in the circuit the 

 image became a broad band of light gradually fading away in intensity. 

 The method of the rotating mirror has been employed by Trow- 

 bridge, who photographed the image given by the mirror, and by com- 

 paring the breadth of successive bright bands, was enabled to deduce 

 the ratios of succeeding half-oscillations. 



In the following experiments the magnetization of steel needles 

 placed in the discharge circuit was made the basis of the method 

 employed. 



Professor Eutherford has shown in a paper on ''A Magnetic Detec- 

 tor of Electrical Waves," that when a steel needle magnetized to satura- 

 tion is placed inside a solenoid and a discharge passed, the magnetization 

 of the needle is reduced, and this reduction is dependent, for any given 

 size of needle, upon the magnetic force in the solenoid and the period 

 of oscillation of the discharge. The effect also varies with the hardness 

 of the steel employed in the needle and with its length and diameter. 

 Thick wires are affected to a less depth than thin wires. 



The needle used in the experiments was composed of fifty-five very 

 fine steel wires, a centimetre and a half in length. These were put 

 together in the form of a compound magnet and insulated from one 

 another by paraffin in order to prevent eddy currents. 



This needle was placed in a groove at the centre of a block of 

 ebonite, around which was fastened an almost complete circle of brass. 

 The brass circle was graduated and provided with a movable arm by 

 means of which any desired arc of the circle might be included in the 

 discharge circuit. The whole was fixed in position before a mirror 

 magnetometer and placed in circuit with a Leyden jar. The circuit 

 contained also an adjustable air-break whose terminals were brass balls 

 a centimetre and a half in radius. 



