120 E. H. Hall—Rotation of the Equipotential Lines 
labels upon them might not be intact, and Mr. G. D. Moore of 
the Harvard class of 1884 has been kind enough to make new 
analyses for me, determining both the copper and the zine in 
specimens furnished him in the form of thin strips such as 
Were used in my own experiments. Mr. Moore found 
Specimen. opper. Zine. 
A SOC Uer Cent. ns 
B 81°08 18°51 per cent. 
C 72°86 ‘ 
D 66°85 33°04 
F 5°87 93°79 
G a trace 99°54 
Specimen E which contained apparently about 50 per cent 
copper was so brittle that I did not succeed in getting it rolled 
into a thin sheet, 
Most, if not all, of these specimens were annealed one or more 
times during the process of rolling. None of the strips exam- 
ined, however, were annealed after the final rolling. All of 
the strips which were used were cut in such a way that the 
arms a, a extended in that direction in which the strips passed 
through the rolls. 
As it is a somewhat troublesome matter to determine accu- 
rately the density of a thin strip of metal and as my immediate 
purpose did not demand great accuracy in this respect, It 
seemed allowable to estimate the density of the alloys from 
their composition. After certain rough experiments the den- 
sity 89 was assumed for the copper, and 7-2 for the zine. 
Assuming, what we know to be not strictly true, that the den- 
sity of an alloy decreases regularly as its amount of zine per 
nd 
unit mass increases, we 
Alloy. Density. 
8°4 
D 8°3 
F 7°3 
The description of particular strips will now be given. 
im UNG. 1) 
Length of main strip when weighed ---.--.--- 4°20 cm. 
Width of main strip when weighed -.-- .----- 1°07 cm. 
Area includipg that of thé arms ..--.-------- 4°98 cm. sq. 
Weight. sssics-. cagaiaees pee, 202 grm. 
Density - 89 
Average thickness from above 00456 cm, 
