Hquipotential Lines of an Electric Current. 421 
which is the method used, cannot be applied with full con- 
fidence. Moreover the latter method gives, at best, only the 
average thickness of the strip, whereas I have heretofore as- 
sumed, and in this article still assume, that the effective thick- 
ness of the strip is the thickness of that part which lies between 
the two arms a, a. My practice of late has been, therefore, 
to determine the average thickness by the weight and density 
method, making use of the best data available, and to estimate 
the thickness between the arms by adding to, or subtracting 
from, the average thickness, according as the calipers indi- 
cated the thickness at that place to be greater or less than the 
average. In the case of several of the strips to be mentioned 
hereafter this correction was somewhat carelessly made, and 
there may be an inaccuracy of four or five per cent. in the 
estimated “effective thickness.’”’ With other strips much 
care was taken in this respect, and it is believed that the un- 
certainty in regard to the density of the metal is the greatest 
source of error in determining the thickness in these cases. 
Particulars will be given as the strips are in turn described. 
The 5th source of error was very troublesome in the ex- 
periments upon certain alloys. The galvanometer was un- 
fortunately so placed that not only the magnitude, but even 
the direction of this effect of the electromagnet might be varied 
when, by any means, the galvanometer-needle was turned a 
few degrees from its ordinary position of rest. 
Copper, Zinc, and their Alloys. 
More than three years ago (B. A. Report, 1881) I found 
that if the rotational power of copper is called —, that of zine 
is +. At the same time a specimen of brass, exact compo- 
sition unknown, had been found to lie between copper and 
zine in this respect, but nearer the copper, having in fact a 
small — rotating power. 
Through the kindness of Prof. Trowbridge and Mr. H. K. 
Stevens, | had at command in the summer of 1883 several 
alloys of copper and zinc in widely varying proportions. 
Specimens of these alloys had been analyzed chemically by 
Mr. Stevens; but as they had lain for some time in an exposed 
position after he had finished his work upon them, I feared 
the 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 
zinc in specimens furnished him in the form of thin strips 
such as were used in my own experiments. Mr. Moore 
found :— 
