A. M. Mayer—Researches in Electro- Magnetism. 195 
Art. XX.—Researches in Electro-Magnetism; by ALFRED M. 
Mayer, Ph.D. 
Tue refined experiments of Weber, Tyndall and Knoblauch 
having fully established the fact of the reversed polarity of a 
bar of bismuth, when subjected to the magnetizing influence 
of a helix, the ‘attention of natural philosophers is naturally 
directed to the necessity of — Ampére’s theory of mag- 
netism to as severe deductive tests as can be applied. Thus, 
from new experiments end lines of research suggested by theo- 
retic deduction, we ma e for such addition to our know- 
ledge as will evolve a eur which will embrace both magnetic 
and diamagnetic phenomena, as completely as Ampére’s beau- 
tiful generalization contains the explanation of the magnetiza- 
tion of iron. 
se ee other experiments, thus suggested, was the follow- 
ing: Ascertain = relative forces of two electro-magnetic cores ; 
one composed 0 — wires, the other of the same number 
of similar wires w lated. 
Theory ‘eidiontins ‘ik the insulated bundle will be found the 
weaker; for, in this case, we have not only the reaction of the 
molecular currents, but also, the reaction of the infinitel “aaa 
currents flowing around each insulated wire. Also, admitting 
the truth of the molecular hypothesis, we — in the unins . 
lated wire-core the interaction of exceedingly small curre 
separated by distances, great when eeripared to their size ; see 
in the insulated wire-core we have, in addition, very large eur- 
rents reacting at distances very small when compared to their 
iameters. 
It was the attempt to solve this problem which led to the 
invention of the experimental method described in this paper; 
for, it will appear further on, that it could not have been at- 
tacked by methods heretofore used. The satisfactory solution 
of this question and the ascertained delicacy and precision of 
the ap 2 ean siaiinged a me to make other determinations, 
which I here present ; not asa finished and neatly rounded = 
of work, but as showing what may be expected from a more 
complete discussion of the method used, and from that perfect 
experimental control of the apparatus which a more extended 
experience will give. 
In my first experiments I adopted the plan of Miiller (Pog- 
gendorft” s Annalen, Bd. LXxrx and Lxxx1t) and tried the forces 
of the different cores, magnetized in one and the same helix, 
by their action on a distant magnetic needle; keeping the cur- 
rent, as far as possible, of the same. intensity during the two 
comparisons. method I found could not serve the purpose 
of measuring forces differing but slightly in intensity ; and the 
