168 E.. H. Hall—New Action of Magnetism 
. . . . . . - 
periment are gold, silver, iron, tin, nickel and platinum. The 
direction is the same in all except iron. 
e extreme irregularity in the results obtained in the early 
n usin 
liability to error from this source, as a low resistance gal- 
vanometer must then be employed, which may easily change 
r 
curately. Even if the specific gravity were the same for all 
the strips, and it probably is not, the value thus obtained for 
the thickness would give only the average thickness, and this 
is by no means the effective thickness. It will be remembered 
that the connections leading to the Thomson galvanometer are 
placed opposite to each other with the width of the metal strip 
between them. e effective thickness is the average thick- 
ness along the line joining these two side connections. Gol 
foil is obtained in sheets ten or twelve cm. square. It will be 
seen further on, that in one case two strips cut from similar po- 
sitions in the same sheet differed in average thickness about 
seven per cent. This being the case it seemed quite possible 
that the effective thickness of any strip, as defined above, may 
