98 Miss H. Brooks on the Damping of the 



then magnetized again to exactly the same amount as before, 

 and replaced in its position at the centre of the circle. The 

 direction of the discharge was reversed, and the arm of the 

 circle moved until the deflexions in the two cases were the 

 same. When this is the case, the ratio of the maximum values 

 of the first and second half-oscillations is given by the ratio 

 of the arcs traversed by the discharge. 



The amount of demagnetization of the needle is approxi- 

 mately proportional to the magnetic force measured by the 

 length of the arc of the circle through which the discharge 

 passes until the needle is more than half-demagnetized, after 

 which the magnetization falls off more slowly, and the curve 

 representing the relation becomes concave with respect to the 

 axis along which the arcs are measured (fig. 3,<x). The circuit 

 was always so arranged that the magnetic moment of the 

 needle was not reduced to more than half its value by the 

 passage of the discharge, and the measurements were thus 

 always made on the straight part of the curve. When the 

 current was very large, the part of the circuit containing the 

 needle had to be shunted. Repeated experiments with and 

 without the shunt when the current was just at its limiting 

 value for use without it, showed that it made no difference in 

 the final relations of the observations. 



It was found rather troublesome in practice to adjust the 

 arcs to give exactly the same deflexion for the two directions 

 of the discharge, and a slight change was made in the method 

 of making the observations which simplified them. The 

 observations made in this way could be reduced by means of 

 a calibration-curve to the exact equivalent of those obtained 

 in the manner described before. 



The needle was magnetized to saturation and placed at the 

 centre of the circle as before, and the discharge passed in the 

 direction in which the second half-oscillation demagnetized 

 the needle ; the deflexion was read, and then, without re- 

 moving the needle or changing the arc of the circle, the 

 discharge was reversed and the increased deflexion noted. 

 By means of the curve (fig. 3. b) showing the relation between 

 the arc and the deflexion, the arcs corresponding to these 

 deflexions were determined, and the damping found, as 

 before, much more accurately than by the first method, 

 where it was necessary to make a number of trials to find 

 the exact arc which would give the required deflexion. 



The first discharge which passes through the circuit causes 

 a reduction of the magnetization, the amount depending on 

 the arc, the period of oscillation, and the resistance. A few 



