€34 MR JAMES RUSSELL ON 



§ 6. Figs. i. and II. show the apparatus used. S S is one of the iron shields fitted 

 in a vertical position on a suitable framework F F, with large rectangular coils C and C, 

 one on each side, after the manner of a Helmholtz tangent galvanometer. The mutual 

 axis of the coils was at right angles to the magnetic meridian, and the value of the 

 transverse field in the central region to be occupied by the shield was determined 

 experimentally in C.G.S. units per unit current in the coils. In order to determine the 

 magnetic field within the shield, a rectangular coil or inductor I) wound with fine wire 

 was placed within the shield and so pivoted that, on releasing a catch K, the inductor 

 was caused to rotate by means of an elastic band half a revolution. This inductor was 

 in connection with a ballistic galvanometer, which was calibrated from day to day 

 for these shielding measurements by the rotation of the inductor D, in the known 

 magnetic field, when the shield was altogether removed. The magnetic axis of the 

 rotating inductor coincided with that of the two large coils C C and C C in its two 

 stationary positions. 



This apparatus was used in connection with two sources of current (E.P.S. cells), 

 so that the transverse and circular fields could be varied independently the one of the 

 other. The accessories were modifications of those in use for the determination of the 

 magnetic properties of iron by the ballistic method arranged to suit the special re- 

 quirements of the experiments. Both fields could be increased by steps from 

 zero to a maximum, reversed at each step, carried from a positive maximum by 

 decreasing steps through zero to a negative maximum, or the maximum currents could 

 during reversals be continuously decreased to zero. The whole was arranged so 

 that one observer could conveniently manipulate in any given sequence the necessary 

 apparatus, and at the same time read the ballistic galvanometer, which w T as sufficiently 

 removed from disturbing fields. A shunted Weston ammeter was used for current 

 measurements, and the long solenoid and measured current method for the calibration 

 of the galvanometer, as also for the determination of the value of the transverse field 

 above referred to. 



§ 7. In the experimental determinations of the shielding ratios, the following 

 magnetic fields require to be known : The transverse field, undisturbed by the presence 

 of the iron shield, due to the coils C C and C C, is known experimentally in terms of 

 the current strength. The circular field which, when acting alone, is the true magnetis- 

 ing force in the iron, is obtained from the known ampere turns per cm. of circumference 

 of the shields.* The other fields are those within the shield, which are determined by 

 the rotation of the inductor D. They are, first, the weakened or shielded field existing 

 within the shield, when the transverse field is maintained by a current in the coils C C 

 and C C ; and second, the negative residual field within the shield due to polarity 

 when the transverse field is withdrawn. In addition to these two fields there is a third 

 magnetic field within the shield due to the overlapped joint and possible want of 



* The longitudinal field is also obtained from the known ampere turns per cm. of length, but is not the true value 

 <. f H acting in the iron, as the demagnetising factor is considerable. 



