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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

dt

R

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.

Yours sincerely,

John TROWBRIDGE, S.D. Harv. Coll. Cambridge, U. S.

INDEX TO VOL. XLIX.

A BSORPTION phenomena, on a

new class of, 323.
Acoustics, researches in, 352, 428.
Allport (S.) on the microscopic struc-

ture and composition of British

carboniferous dolerites, 479.
Aniline derivatives, on, 21.
Attfield (Dr.) on the spectrum of car-

bon, 106.
Audition, on new facts in the physio-

logy of, 360.
Aurora, on the spectrum of the, 65,

249, 410, 481.
Belt (T.) on the geology of the

Steppes of Siberia, 479.
Benoit (R.) on the electrical resist-

ance of metals, 78.
Bezold (W. von) on the mixture of

colours in binocular vision, 78.
Blondlot (R.) on the determination

of the quantity of magnetism in a
magnet, 482.
Böttger (R.) on the conservation and

properties of a plate of palladium
saturated with hydrogen by elec-

trolysis, 80.
Books, new :--Spottiswoode's Polari-

zation of Light, 72; Ruchonnet's
Exposition géométrique des Pro-
priétés générales des Courbes, 225;
Ruchonnet's Éléments de Calcul
approximatif, 227; Williamson's
Integral Calculus, 319; Jevons's
Principles of Science, 400; Wein-
hold's Introduction to Experimen-

tal Physics, 404.
Boron, on the specific heat of, 161,

290.
Bosanquet (R. H. M.) on the mathe-

matical theory of Hamilton's string
organ, 98.

Bouty's (Prof. E.) studies on mag-

netism, 81, 186 ; on the magneti-
zing-function of tempered steel,

320.
Broun (J. A.) on the sun-spot period

and the rainfall, 474.
Camacho (J.) on a new electromag-

net, 244.
Capron (J. R.) on the comparison of

some tube and other spectra with
the spectrum of the aurora, 240,

481.
Carbon, in the modern spectroscope,

on, 24, 104, 106; on the specific

heat of, 161, 175, 276.
Cockle (Sir J.) on primary forms,

134.
Colour, on combination of, by means

of polarized light, 146.
Coniferine, researches on, 229.
Cryohydrates, on the, 11, 206, 266.
Culley (R. S.) on Camacho's new

electromagnet, 332.
Curtis (Prof. A. H.) on extraordinary

reflection, 366,
Diamagnetism, on the experimental

determination of, by its electrical

inductive action, 246.
Diamond, on the specific heat of,

175.
Diffraction, on the projection of the

Fraunhofer lines of, on a screen,

142.
Dolerites, on the microscopic struc-

ture and composition of British

carboniferous, 479.
Draper (Prof. J. C.) on the projec-

tion of the Fraunhofer lines of dif-
fraction and prismatic spectra on a
screen, 142.
Electric conducting-power of the

chlorides of the alkalies, alkaline Goldstein (E.) on the spectra of gases,
earths, and nitric acid in aqueous

333, 448.
solutions, on the, 417.

Graphite, on the specific heat of,
Electric discharge, on a new method of 276.

investigating the composite nature Grotrian (O.) on the electric conduct-
of the, 47.

ing-power of the chlorides of the
Electrical problems, on graphical me- alkalies, alkaline earths, and nitric
thods of solving certain, 368. acid in aqueous solutions, 417.

resistance of metals, researches Guthrie (F.) on salt-solutions and
on the, 78.

attached water, 1, 206, 266.
Electricity, on the flow of, in a uni- Haarmann (W.) on coniferine and its

form plane conducting surface, conversion into the aromatic prin-
385, 453.

ciple of Vanilla, 229.
Electrolysis of certain metallic chlo-

Hamilton's string-organ, on the ma-
rides, on the, 425.

thematical theory of, 98.
Electrolytes, experimental investiga- Hautefeuille (P.) on the solution of

tion of the work of the current in hydrogen in metals, and on the de-
the interior of, 417.

composition of water by iron, 413.
Electromagnet, on a new, 244, 332, Heat, on the theory of, 154; on the
484.

mechanical equivalent of, 416.
Equilibrium and initial and steady Herschel (Prof. A. S.) on the spec-

motions, on general theorems re- trum of the aurora, 65.
lating to, 218.

Holtz machine, experiments with the,
Etna, on the eruption of, on the 29th 411.
August, 1874, 126.

Hydrogen, on the solution of, in
Explosive bodies, on the decomposi- metals, 413.
tion of certain, 157.

Ice, on permanent, in a mine in the
Flames, on the reflecting-power of, Rocky Mountains, 77.
50; on the reflection of sound

Iron, on the decomposition of water
from, 428.

by, 413.
Force, on Helmholtz's memoir on Kohlrausch (Prof. F.) on the electric
the conservation of, 377.

conducting-power of the chlorides
Foster (Prof. G. C.) on graphical of the alkalies, alkaline earths, and

methods of solving certain electri- nitric acid in aqueous solutions,
cal problems, 368; on the flow of 417.
electricity in a uniform plane con- Light, on polarization by diffusion of,
ducting surface, 385, 453.

50.
Galton (F.) on statistics by intercom- Liquids, on some phenomena con-

parison, with remarks on the law nected with the boiling of, 432.
of frequency of error, 33.

Lockyer (J. N.), spectroscopic notes
Gases, on the liberation of, from su-

by, 233, 320.
persaturated solutions, 157, 302; Lodge (O. J.) on the flow of electri-
on the work that may be gained city in a uniform plane conducting
during the mixture of, 311; on the surface, 385, 453.
coefficient of friction of, 332; on Lyman (Prof. C. S.) on Venus as a
the spectra of, 333, 448.

luminous ring, 159.
Geological Society, proceedings of Magnet, on the determination of the
the, 73, 152, 237, 326, 408, 478.

quantity of magnetism in a, 482.
Gernez (D.) on the liberation of gases Magnetism, studies on, 81, 186; on,

from their supersaturated solutions 243.
and the decomposition of certain Mallet (R.) on volcanic energy, 144;
explosive bodies, 157.

on expansion by refrigeration, 231.
Gladstone (Prof. J. H.) on the elec- Mayer (Prof. A. M.) on the compo-

trolysis of certain metallic chlorides, site nature of the electric discharge,
425.

47; researches in acoustics by, 352,
Glaisher (J.W.L.) on partitions, 307. 428.

scope, 472.

Metallic chlorides, on the electrolysis Silvestri (Prof. O.) on the eruption of
of certain, 425.

Etna on the 29th August, 1874,
Metals, on the electrical resistance of, 126.

78; on the solution of hydrogen Soret (J.-L.) on polarization by dif-
in, 413.

fusion of light, 50.
Mills (Dr. E. J.) on aniline deriva- Sound, on the transmission of, 151;
tives, 21.

on the law connecting the pitch of
Molecular structure, on the evidence a, with the duration of its residual
of variation in, 235.

sensation, 352; on the reflection
Moon (R.) on Helmholtz's memoir of, from flames and heated gases,

on the conservation of force, 377. 428.
Moutier (J.) on the expression of the Spectra, on the projection of pris-

work relative to an elementary matic, on a screen, 142; of gases,
transformation, 154.

on, 333, 448.
Nitric acid, on the electric conduct- Spectroscopic notes, 233, 320.

ing-power of, in aqueous solutions, Spectrum-analysis, researches in, 326.
421.

Spectrum of carbon, on the, 24, 104,
Obermayer (Capt. A. von) on the de- 106.

pendence of the coefficient of fric- Spottiswoode (W.) on combination of
tion of atmospheric air on tempe- colour by means of polarized light,
rature, 332.

146; on a new revolving polari-
Palladium, on the conservation and

properties of a plate of, saturated Statical theorem, on a, 183.

with hydrogen by electrolysis, 80. Statistics by intercomparison, on, 33,
Partitions, on, 307.

Steel, on the magnetization of, 90,
Polariscope, on a new revolving, 329.
472.

Steel magnets, on the theory of,
Polarization by diffusion of light, on,

199.
50.

String organ, on the mathematical
Primary forms, on, 134.

theory of Hamilton's, 98.
Puluj (H. J.) on a lecture-room ap- Sun, on the spectrum of the, 326.

paratus for the determination of Sun-spot period and the rainfall, on
the mechanical equivalent of heat, the, 474.
416.

Telegraphy, on the general theory of
Rainfall, on the sun-spot period and

duplex, 108.
the, 474.

Tiemann (F.) on coniferine and its
Rayleigh (Lord) on a statical theo- conversion into the aromatic prin-

rem, 183; on general theorems ciple of vanilla, 229.
relating to equilibrium and initial Toepler (Prof. A.) on the experimen-
and steady motion, 218; on the tal determination of diamagnetism,
work that may be gained during

246.
the mixing of gases, 311.

Tomlinson (C.) on the action of solids
Reflection, on extraordinary, 366. in liberating gas from solutions,
Refrigeration, on expansion by, 231. 302; on somephenomena connected
Rossetti (F.) on experiments with the with the boiling of liquids, 432.
Holtz machine, 411,

Trève (A.) on magnetism, 243.
Royal Society, proceedings of the, Tribe (A.) on the electrolysis of cera
144, 229, 320, 474.

tain metallic chlorides, 425.
Salt solutions and attached water, on, Troost (L.) on the solution of hydro-
1, 206, 266.

gen in metals, and on the decom.
Schwendler (L.) on the general theory position of water by iron, 413.
of duplex telegraphy, 108.

Trowbridge (J.) on Camacho
Sea-water, on the freezing of, 17, electromagnet, 484.
Silicon, on the specific heat of, 161, Tyndall (Prof. J.) on the transmis.
294.

sion of sound, 151.

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