96 



Miss H. Brooks on the Damping of the 



the current being measured by the deflexion of the magneto- 

 meter due to the demagnetization of the detector-needle, 

 instead of by the direct deflexion of a magnet at its centre. 



Fig. 2. 



Al 



The discharge-circuit is shown in fig. 2. A and B are leads 

 from the terminals of a large Wimshurst machine which sup- 

 plied the current. C is a key by means of which the direction 

 of the discharge could be reversed; and at D is an air-break 

 whose terminals were brass balls 2*1 cms. in diameter, and 

 whose distance could be varied at will. E is the leyden-jar, 

 to the outer coating of which a wire is led from one terminal 

 of the air-break, while the other terminal is connected through 

 the circle to the inner coating of the jar E. 



The magnetic field due to the wires leading out from the 

 circle was found to affect the results. To prevent this 

 the wires were carried out for some distance parallel to the 

 needle and enclosed in a glass tube, which was in turn sur- 

 rounded by a brass cylinder connected to earth, so that all 

 magnetic effect due to this field was done away with. The 

 remainder of the circuit was at too great a distance from the 

 needle to have any effect on it. In this way it was ensured 

 that the demagnetization of the needle w'as entirely due to 

 the current through the arc of the brass circle. Since the 

 magnetic field at the centre of a circle due to an arc of 



length I is given by H= -|, where 7 is the current, we see 



