McKAY. — HALL EFFECT AND CURRENT DENSITY IN GOLD. 371 



Mat 14. Plate 4. 



In the case of the gold leaf no value for the thickness has been given. 

 When a calculation of the thickness is made from the dimensions of the 

 strip (6.2 cm. long and 1.5 cm. wide) and its resistance (3.8 ohms), 

 one obtains 2.3 X 10 -6 cm. as a result. This is based on the value 

 2.15 X 10 -6 ohms as the specific resistance of gold. This value is 

 probably not quite accurate, since it was the custom of the gold-beater 

 from whom the leaf was obtained to use an alloy containing about 96 

 per cent of gold, 2 per cent of silver, and 2 per cent of copper. It was 

 not at all uniform in thickness, as could be seen even with examination 

 by the naked eye. Again, in very thin sheets of metal the electrical 

 resistance increases very rapidly as the thickness is diminished.* The 

 above value of the thickness is therefore not given as in any sense accu- 

 rate. When it is used to estimate the Hall effect in the case of Plate 4, 

 the value 0.000181 is obtained, that is about one fourth the value obtained 

 in the case of the thicker gold plates 1, 2, and 3. 



It is also to be remembered that Moreau, quoted above, found in 

 the case of silver (and nickel) that the Hall effect diminished rapidly 

 with thicknesses below 50 millionths of a millimeter. It seems probable 

 that the Hall effect in gold diminishes in the same way. 



Two additional sets of observations were made on Plate 3, the poten- 

 tiometer method (see Fig. 4) being adopted to measure the main cur- 

 rent. As in the case of the later measurements on Plate 4 the plate 

 was kept in the same position during a set of consecutive measurements, 

 including changes from weak to strong main current, or vice versa; but 

 frequent readings of the magnetizing current were taken, and thus the 

 changes of the magnetic field observed. 



* Ettingshausen, Sitzungberichte Akad. Wicn, 81, 446 (1880); A. C. Longden, 

 Physical Review, 11, 84-94 (1900) ; J. J. Thompson, Cambridge Proe. (2), 9, 120- 

 122 (1901). 



