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Mr. A. K. Chapman on the 



measuring the small potential differences obtained in a plate 

 0*0398 cm. thick. The Corbino effect rises to a maximum 

 at a field of about 4500, and decreases rather rapidly to 

 fields of 9000 lines, which is as far as it was practicable 

 to carry the readings. It is likely that the Hall effect also 

 has a maximum at approximately 4000, and it certainly 

 decreases for increasing field at nearly the same rate as the 

 Corbino effect, the two curves being almost parallel from 

 6500 to 9500. 



The Influence of Thickness. — The theory mentioned above 

 predicts that the Corbino effect is independent of the thickness 

 of the disk employed. This theory has been confirmed in 

 this one respect, within the limits of experimental error, for 

 three different nickel disks of quite different thicknesses. 

 They were supplied by Messrs. Baker & Company, and were 

 of thicknesses 0*0505, 0*1526, and 0*20315 cm. respectively. 

 Here one must take into account the change in mutual 

 inductance between the coils and the disks of varying 

 thickness. To obtain an accurate measure of this change, 

 the method used in the previous paper was employed. The 

 coils were placed against a circular plate of brass, 5 cm. in 

 diameter and having a hole of the same size as the central 

 wire of the other disks drilled in the centre. A sector of 

 small angle was cut out of this disk, to the edges of which 

 heavy leads were soldered. The coil and disk were then 

 placed between the poles of the magnet and their mutual 

 inductance compared with a known standard. A heavy 

 sheet of mica was then interposed between the coil and disk, 

 and the mutual inductance again measured. In the first 

 instance the distance from the centre of the coil to the disk 

 was 0*0438 cm., while the mutual inductance proved to be 

 21000 e.m.u. ; in the second the distance was 0*1317 cm. 

 and the inductance 17500 units. Assuming, then, that the 

 change of inductance with separation is linear for the changes 

 used with the three disks above, we may calculate the 

 inductance between the coils and each disk. Taking the 

 inductance measured with the brass disk of 0*0894 cm. 

 thickness as standard, we find : — 



The inductance increase for the thin disk 3*90 per cent. 

 ,, ,, decrease „ medium ,, 6*32 ,, 



„ thick „ 11*38 



Below are the values of 8/F for the three disks, where 8 

 is the change in balancing inductance, measured in degrees 



