MEASUREMENT OF PERMEABILITY 267 



A represents the solenoid placed vertical. B is the magneto- 

 meter nearly level with the upper end of A, and at a distance from 

 it so short that the action on the needle is almost entirely due to 

 the upper pole. The rod must be adjusted to the height at which 

 the action is a maximum. The magnetising current through the 

 solenoid is supplied by the cells on the right, and on its way to 

 the amperemeter G and the commutator F it passes through the 

 adjustable liquid resistance H, which consists of a tall glass jar 

 with three discs of zinc immersed in a dilute solution of zinc 

 sulphate. 



It will be seen from the figure that the circuit from one 

 terminal of the battery passes through the top disc on its way to 

 G and the solenoid, while the other terminal is connected by an 

 insulated wire to the bottom disc. The other end of the solenoid 

 is connected to the middle disc, which is movable up and down. 

 If the middle disc touches the top one, the current flows through 

 the jar and none goes through the solenoid. As the middle disc 

 is lowered the current through the solenoid increases and is a 

 maximum when the middle disc touches the bottom one. On its 

 way to the solenoid the current passes through the " compensating 

 coil " E, which is adjusted in such a position that when there is no 

 rod within the solenoid the action of E on the magnetometer is 

 just equal and opposite to that of the solenoid. The magnetic 

 action of the solenoid itself may therefore be left out of account. 



In order that the earth's field may be eliminated a second 

 solenoid is wound round the first and a constant current from a 

 cell C is passed through it and through a resistance D so adjusted 

 that the field due to the constant current within this second 

 solenoid is equal and opposite to the earth's vertical field. 



When a specimen is to be tested it is necessary in the first 

 place to demagnetise it. This is effected by the " method of 

 reversals." The rod is placed within the solenoid and the full 

 current is put on by lowering the middle plate in the liquid 

 resistance to contact with that at the bottom. The commutator 

 F is then rapidly revolved, so as to alternate the field within the 

 solenoid. While the rapid alternation is going on the middle 

 plate is slowly raised through the liquid to the top so that the 

 amplitude of the alternating current slowly decreases from its 

 largest value to zero, and the rod is found to be completely 

 demagnetised by the process. This method serves also to adjust 

 the current through the outer solenoid to the value required 

 to neutralise the earth's field, for the reversals only completely 

 demagnetise the rod when the field is finally reduced to zero. It 

 is useful to employ for this adjustment a special rod of annealed 

 soft iron. 



The specimen having been demagnetised, let a current of A 

 amperes be established in the solenoid. If there are n turns per 

 centimetre the field due to it is H' = 4>TrnA/lO. 



