A. M. Mayer—Researches in Electro-Magnetism. 201 
pass needle, and as 8’ or even less of deflection can be accurate- 
ly read it follows that a difference of ;,;;th in the magnetic 
force of the two cores can be detected and measured by this 
method. This example will convey an idea of the delicacy and 
accuracy of this plan of measurement, which no doubt can be 
increased by the careful selection of the proper force of current, 
distance of helices and other related functions of the apparatus. 
It thus appears that this result sides with the theory of Am- 
pére, and also, to my mind, points to that postulate of the atomic 
hpyothesis which states that the atoms of bodies are separated 
from each other by distances which are great when compared to 
the size of these atoms. 
Altempts to determine what thickness of tube in fraction of tts di- 
ameter, 1s equal to a solid cylinder of the same diameter.—According 
to the theory of Ampére, the magnetization of a bar of steel, or 
of an iron core subjected to a helical current of electricity, is due 
to the surface currents, which are the resultants of the interac- 
tion of all the molecular currents in the interior of the bar. It 
is therefore exceedingly interesting in a theoretic point of view 
to determine accurately what thickness of tube is equivalent to 
a solid cylinder of the same length and diameter. The experi- 
ments of Barlow, Harris, de Haldat, and of Du Moncel have 
shown the truth of this deduction. 
There is evidently a certain thickness required to develope 
an equivalent action, but a certain Tatton of the interior of an 
electro-magnetic core can be removed without diminishing its 
force when in a condition of “ saturation.” This determination 
will therefore inform us, (accepting the hypothetical theory of 
Ampére), what central currents are prevented by their inter- 
action from affecting the resultant surface currents of the bar; 
and it appears to me that we should express the result in terms 
of the central cylinder which can be removed and not of a certain 
thickness of tube which remains. 
Out of well annealed soft iron a series of tubes, 10 in. long, 
were turned so that they fitted closely into each other; into the 
innermost tube slid a solid iron core of ‘83 in. diameter. The 
diameter of the outer tube was1‘68 in. With this construction 
of core, tubes of various thicknesses could be rapidly produced 
and compared in effect. Two such cores were construct 
Ex. 12. uninsulated wire-core in W helix; in E helix 
was placed the combination 6f tubes and core. Needle brought 
to such a position that it stood at 0° when circuit was open or 
closed. T. G. 46°. 
Hx. 13. Removed inner core and tubes, and drilled out the 
innermost remaining tube until the thickness of the combina- 
tion tube was j of its diameter. Current passed. Needle re- 
Am. Joun. Sci—Szconp Series, Vou. L, No. 149.—Sepr., 1870. 
13 
