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This relation, in general very complicated, is simplified in a special case, as we will show.
Let us consider the potential V of a pole P. It is known that this potential has for its value in absolute measure the opening of the cone under which the pole P regards the current. If, then, the circuit B starts from negative infinity to reach the pole and then remove thence to positive infinity, the potential varies by the quantity 47.
From this it follows that, in the above-indicated conditions of displacement, V-V, is a constant quantity, and equal to 4. for all the poles; consequently it can be put as a factor, which gives
4 Su=-R 4TM,
designating by M the total magnetism of the arrangement.
In a long magnet the magnetism can be regarded as collected in the vicinity of the extremities; therefore if the coil be placed on the middle portion of such a magnet and the latter be afterwards removed to a great distance, the conditions will be sensibly those of the preceding theory. It hence follows that the quantity of the current can serve for measuring the total magnetism of the half of a bar, provided that the bar be not too short—that is to say, that its polar distance be not less than 8–10 centims.
We see also that the current is independent of the diameter of the coil, if this diameter is a small fraction of the length of the bar. The last proposition was experimentally verified by Faraday and Lenz, and more recently by M. Gaugain.--Comptes Rendus de l' Académie des Sciences, vol. lxxx, pp. 653–656.
ON CAMACHO's NEW ELECTROMAGNET. To the Editors of the Philosophical Magazine and Journal, GENTLEMEN, In the April Number of your Magazine Mr. R. S. Culley states that he had in his possession, in 1852, a magnet similar in principle to that of M. Camacho, which was invented by the late Richard Roberts. I take the following extract from a paper on the Construction of Galvanic Magnets, by John B. Zabriskie, M.D., published in the American Journal of Science and Arts for July 1839. After dwelling upon the difficulty of “saturating with magnetism large masses of iron,” he continues, “ but if we divide perpendicularly each extremity of a large magnet into four equal parts and wind each part separately, there will be no difficulty in completely saturating the whole.” This seems to me to be the same idea upon which both Mr. Roberts and M. Camacho proceeded to construct their magnets.
John TROWBRIDGE, S.D. Harv. Coll. Cambridge, U. S.
INDEX TO VOL. XLIX.
A BSORPTION phenomena, on a
new class of, 323.
ture and composition of British
carboniferous dolerites, 479.
logy of, 360.
249, 410, 481.
Steppes of Siberia, 479.
ance of metals, 78.
colours in binocular vision, 78.
of the quantity of magnetism in a
properties of a plate of palladium
zation of Light, 72; Ruchonnet's
tal Physics, 404.
matical theory of Hamilton's string
Bouty's (Prof. E.) studies on mag-
netism, 81, 186 ; on the magneti-
and the rainfall, 474.
some tube and other spectra with
on, 24, 104, 106; on the specific
heat of, 161, 175, 276.
of polarized light, 146.
determination of, by its electrical
inductive action, 246.
Fraunhofer lines of, on a screen,
ture and composition of British
tion of the Fraunhofer lines of dif-
chlorides of the alkalies, alkaline Goldstein (E.) on the spectra of gases,
Graphite, on the specific heat of,
investigating the composite nature Grotrian (O.) on the electric conduct-
ing-power of the chlorides of the
resistance of metals, researches Guthrie (F.) on salt-solutions and
attached water, 1, 206, 266.
form plane conducting surface, conversion into the aromatic prin-
ciple of Vanilla, 229.
Hamilton's string-organ, on the ma-
thematical theory of, 98.
tion of the work of the current in hydrogen in metals, and on the de-
composition of water by iron, 413.
mechanical equivalent of, 416.
motions, on general theorems re- trum of the aurora, 65.
Holtz machine, experiments with the,
Hydrogen, on the solution of, in
Ice, on permanent, in a mine in the
Iron, on the decomposition of water
conducting-power of the chlorides
methods of solving certain electri- nitric acid in aqueous solutions,
parison, with remarks on the law nected with the boiling of, 432.
Lockyer (J. N.), spectroscopic notes
by, 233, 320.
luminous ring, 159.
quantity of magnetism in a, 482.
from their supersaturated solutions 243.
on expansion by refrigeration, 231.
trolysis of certain metallic chlorides, site nature of the electric discharge,
47; researches in acoustics by, 352,
Metallic chlorides, on the electrolysis Silvestri (Prof. O.) on the eruption of
Etna on the 29th August, 1874,
78; on the solution of hydrogen Soret (J.-L.) on polarization by dif-
fusion of light, 50.
on the law connecting the pitch of
sensation, 352; on the reflection
on the conservation of force, 377. 428.
work relative to an elementary matic, on a screen, 142; of gases,
on, 333, 448.
ing-power of, in aqueous solutions, Spectrum-analysis, researches in, 326.
Spectrum of carbon, on the, 24, 104,
pendence of the coefficient of fric- Spottiswoode (W.) on combination of
146; on a new revolving polari-
properties of a plate of, saturated Statical theorem, on a, 183.
with hydrogen by electrolysis, 80. Statistics by intercomparison, on, 33,
Steel, on the magnetization of, 90,
Steel magnets, on the theory of,
String organ, on the mathematical
theory of Hamilton's, 98.
paratus for the determination of Sun-spot period and the rainfall, on
Telegraphy, on the general theory of
Tiemann (F.) on coniferine and its
rem, 183; on general theorems ciple of vanilla, 229.
Tomlinson (C.) on the action of solids
Trève (A.) on magnetism, 243.
tain metallic chlorides, 425.
gen in metals, and on the decom.
Trowbridge (J.) on Camacho
sion of sound, 151.