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Fig. 2.—Soft iron Ellipsoid of Revolution cut in equatorial plane and interface

polished. Area of surface of contact=1.767 square centim.

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Induction, B. C.G.S.

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Fig. 3.—Soft iron Ellipsoid of Revolution. Length (after cutting) = 22.493 centim.

Least diameter=1.5 centim. Surfaces of contact plane and polished.

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200 250 300

350 Magnetizing Field, H. C.G.S.

per cent.

Fig. 3 represents the magnetization-curve and the demagnetizing force of the ellipsoid.

Each observation was repeated several times, and the mean value taken as the true value. The weights, however, never differed by more than 1 or 2 per cent. for the same magnetizing current.

It appears therefore that the present method of measuremeat has great advantages over the ballistic method used by Bosanquet and Threlfall, for Bosanquet obtained sometimes for the same induction weights differing by nearly 20

The apparatus as above described (or any apparatus of the S. P. Thompson's "permeameter"* type) may therefore be used for the accurate measurement of magnetic induction in uniformly magnetized bars, the essential conditions being that the contact surfaces are plane, that the upper bar can be finely adjusted in position and very rigidly fixed, and that the contact surfaces are as nearly as possible flush with the ends of the magnetizing-coils, space being left for the small guiding-ring on one of the hars. The lower bar must also be guided, and for this it is sufficient that its lower end pass freely through a ring properly adjusted in position.

A few experiments were made with the half-cores separated by very thin sheets of silver. Two sheets were used, of thicknesses 1.5 and 8 hundredths millim, respectively, and it was found that the introduction of a sheet always caused a diminution of both magvetization and tractive force, greater with the thicker sheet. No increase similar to that found by Wassmuthf at low inductions was observed.

In conclusion, I wish to express my obligations to the late Prof. Kundt and Drs. du Bois and Rubens for the interest they took in my work, and the help they gave me in various ways. Berlin Phys. Inst. d. Univ.,

Jan. 1895,

* Journ. Soc. Arts, Sept. 12, 1890,
L. c. p. 336.

,

XXV. On the Influence of the Dimensions of a Body on the

Thermal Emission from its Surface. By ALFRED W.
PORTER, B.Sc., Demonstrator of Physics, University
College, London".
N discussing the rate at which heat passes outwards from a

conducting body into the medium in which it is immersed, it is usually assumed that (for small excesses of temperature of the body above its surroundings) it may be taken as proportional to the excess of temperature. This law is followed, as far as is known, if the body is in a vacuum and loses heat simply by radiation between its own and surrounding surfaces. If, however, it is immersed in a medium, such as air, which carries away heat by conduction and convection as well as by radiation, the problem becomes more complicated, and to assume that the whole effect may be treated as radiation only does not give results which are even an approximation to those obtained experimentally. For example, on this assumption the amount passing outwards from unit area of the surface per second per unit excess of temperature (i. e. the “emissivity) should be independent of the size of the body. Results obtained by Péclet from experiments on cylinders and spheres of different sizes show that this constant depends materially upon the sizes of the bodies experimented upon. Péclet's formulæ connecting the rate of emission (exclusive of the radiation effect) with the radius (r) for 0° C. excess are :

•0382

For a horizontal infinitely long } 2:058+

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For a sphere

•13 1.774 +

10

in which kilogrammes, metres, bours are the units employed. Other formulæ are given by him which it is unnecessary to quote here.

These results do not seem to have attracted much notice, as they are not given in any of the text-books of Physics. They do, however, appear in a book compiled for practical men by Box t, from which the formula for the sphere is quoted, on * Communicated by the Physical Society: read January 11, 1895.

+ A Practical Treatise on Heat for the use of Engineers and Architects,' by Thomas Box. (London, E. and F. N. Spon, 2nd edition, 1876.) This book does not appear to be generally known physicists; and, judging from recent references to it, I gather that stilf less is it realized that the author's data are to a great extent obtained from Péclet, although he states the source to which he is indebted in his preface.

Professor Ayrton's authority, in Everett's Illustrations of the C.G.S. System of Units' (1891 edition, p. 133) *.

Experiments on thin wires by Messrs. Ayrton and Kilgour have confirmed the fact that the emissivity can be expressed empirically through a considerable range of radius in the form given by Péclet; and experiments on rods which have been in progress in this Laboratory since 1891, an account of which was read by Mr. Eumorfopoulos before the Physical Society on the same day as this paper, lead to the same conclusion.

It was these that first called my attention to the subject; and in order to account approximately for them and the results elsewhere obtained, I propose here to examine the results of supposing the loss to only in part follow the law of radiation, the remainder being assumed to follow the law of conduction.

The rate of loss due to radiation will be proportional to the excess of the temperature of the body above that of its enclosure, and if we reckon temperatures from that of the enclosure, we may write the rate due to this cause

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* A mistake occurs in Everett as well as in a paper on the same subject published later by Professor Ayrton and Mr. Kilgour in the Phil. Trans, for 1892, in which Everett's statement is quoted. The formulæ are given in Box as

-307
•421+ for the cylinder

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and

1.0476 3634+

for the sphere,

r in which the units are the pound, foot, and hour, the radius being however in inches. Translated into C.G.S. units, they become

1.061 Cylinder : •572+ x 10–

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In Everett, and in Ayrton and Kilgour's paper, the latter appears as

0003609 0004928+

Further, it is not clear from Everett whether the formula he gives refers to air-effect plus radiation or to one of these alone. The specification of “ blackened sphere "would lead one to suppose that either the total effect or else the radiation only is meant, since the air-effect has been shown to be independent of the nature of the surface. On reference to Péclet, the formula is seen to represent the air-effect alone.

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