Imágenes de páginas

VII. On a simple Condensing Collector for Frictional Electrical Machines. By SAMUEL ROBERTS, Esq.*


HE prime conductor of a frictional electrical machine is one armature of a condenser, the air being the dielectric, and the more or less conducting objects in the neighbourhood constituting the second armature. This obvious consideration suggests the form of a collector suitable for developing the full length of spark which the machine is capable of giving. Winter's ring aims at an enlargement of the conductor with as little dissipation as possible, and requires accurate and expensive workmanship. The exterior armature remains irregular and variable in the extreme; and we depend on the extent of the intervening dielectric for rendering the effect of these irregularities small. If we control the form of the exterior armature as well as that of the interior one, we really make an artificial condenser or Leyden battery, though the dielectric may still be air. I revert then to the old form of apparatus, in which, for medical purposes, the prime conductor was superseded by a Leyden jar; but as my object is not to accumulate a large quantity of electricity, but to produce long and frequent sparks, to obtain, in fact, the ordinary phenomena of a prime conductor armed with a ring and charged to a high potential, my Leyden jar is of minute dimensions; and in this consists what I conceive to be the novelty. The result of my experiments was so satisfactory that I venture to communicate it. The apparatus is very effective, of insignificant cost, and may be applied to any machine at


For a small machine with a cylinder 6 inches in diameter and 8 inches long, I take a glass tube inch in diameter and 13 inches long, hermetically sealed at one end. A fine copper wire, gauge 25, is inserted; and a piece of paper 2 inches broad is wrapped four or five times round the closed end. The open end with a little wire projecting is to be applied to the prime conductor; and the outer coating must be in good electric connexion with a metallic ball by means of a similar wire twisted once round the paper. I obtained in this way brilliant zigzag sparks 4 inches in clear length.

The charge of the tube at a given potential depends on the interior surface of the tube, and in part on the exterior surface, as well as on the wire and coating. Hence a tube of inch diameter and inch bore will give a spark of considerable volume, which may occasion a disagreeable shock. We can, however, modify the effect indefinitely; and this is a great advantage of the arrangement. The outer coating may be made * Communicated by the Author.

Phil. Mag. S. 4. Vol. 47. No. 309. Jan. 1874.


50 On a Condensing Collector for Frictional Electrical Machines. movable, or can be expanded and contracted by simple sliding. The interior wire should not quite reach the end of the tube, because this is apt to be imperfectly annealed and easily cracks. Neither should the outer coating, if metallic, be tight and rigid; for I found that a tight brass slider, at once carrying the tube and forming the exterior coating, caused speedy rupture by spontaneous discharge. After several failures, the arrangement above described enabled the tube to bear a high charge without damage.

We may, however, with some advantage dismiss the conductor (which is commonly ill adapted to retain electricity at so high a potential), and substitute for it a long open tube mounted at one end with points and at the other with a brass knob, the points and knob communicating through the tube by a fine wire. I constructed a collector of this kind with a portion of a barometer-tube of inch diameter and inch bore, 19 inches long, and coated near the middle with tinfoil for 1 inch. This worked remarkably well, giving sparks 4 inches long; but a little additional length and a somewhat larger bore would be an improvement. For a large machine a piece of steam-gauge tubing would be suitable. The stem which carries the tube may be metallic, or of wood with a wire passing along it.

I observed that the full power of the machine was not yet exhibited, and therefore made a wooden point-holder supported by a glass stem. In the upper surface is an aperture in which, with a cork, a Leyden tube can be fixed in an upright position. A piece of wire twisted about the outer coating (which may be of paper rubbed with plumbago) communicates with a metallic knob. We may also fix the closed end of the tube in the aperture and connect the interior wire with the metallic ball. The free electricity on the covered surface of the tube is thus got rid of to some extent. This apparatus enabled me to obtain fine sparks in air between balls over 5 inches apart. The electricity, however, was frequently discharged over the glass of the cylinder; and it seemed that the limit allowed by the conditions of the insulation was nearly attained. I believe this result is remarkable for so small a machine.

The apparatus is so simple, compact, and effective that it should certainly be associated with every frictional electrical machine. Leyden jars are made use of in connexion with Holtz's machine with wonderful success; but the smallest I have seen are altogether too large for ordinary machines, and, as I have said, the pith of the matter is that the condenser must be charged and discharged frequently at the highest potential the machine can give. Of course it is desirable to varnish the tubes and otherwise take the usual precautions.


VIII. On the Molecular Changes that accompany the Magnetization of Iron, Nickel, and Cobalt. By W. F. BARRETT, Professor of Physics in the Royal College of Science, Dublin*.


HE magnetization of iron is accompanied by certain molecular changes in the metal which are well known to physicists. The object of this paper is an endeavour to extend our knowledge of some of these changes. Further inquiry in this direction seems to be needed as presenting one avenue of approach to a better insight of what may be termed the "molecular architecture" of a magnet.

The wonderful transition of iron from an ordinary to a magnetized condition makes no alteration in the appearance, the temperature, the weight, or the total bulk of the iron; but it is associated with the changes alluded to, which are briefly as follows:

1st. The act of magnetization causes a slight increase in the length, and a corresponding diminution of the breadth of an iron bar a fact discovered by Mr. Joule in 1842, confirming the previous observations of MM. Gay-Lussac and Wertheim, that there was no alteration in the total volume of the iron. This elongation, however, does not occur when the iron is submitted to a definite longitudinal strain; and when the strain is still greater, the iron invariably shortens when magnetized.

2nd. A sound is emitted by the iron on magnetization and again on demagnetization. This was revealed by Mr. Page in 1837, and studied by many physicists subsequently. In iron wires the sound or clink seems composed of two distinct noises, one of which intensifies by a moderate strain, but is destroyed and the whole sound enfeebled by a still higher strain.

3rd. M. Wiedemann has proved that an iron wire hung in the centre of a helix and twisted is more or less untwisted when a current traverses the helix and magnetizes the wire. M. Matteucci has shown that twisting a magnet lessens its force, but stretching a magnet slightly adds to its power; and according to M. Guillemin, a strip of iron bent by its own weight is partly straightened by magnetization.

4th. The conduction of heat in magnetized iron is greater across than along the magnetic axis a fact discovered by Dr. Maggi, and enlarged by Sir W. Thomson, who has shown that

Communicated by the Author, having been read before the British Association at Bradford, September 1873.

† Nevertheless it is stated that repeated magnetization and demagnetization raises the temperature of an iron bar.

A refined and capital method of exhibiting this is described by Mr. Gore in the Proceedings of the Royal Society' for January 28, 1869, vol. xvii. p. 265.

its precise analogue is to be found in the conduction of electricity in magnetized iron and nickel.

5th. A bar of wrought iron is more easily magnetized in the direction of its fibre; and steel once magnetized in a given direction and then demagnetized, is more readily magnetized in its first direction than in any other-a fact first pointed out by M. Marianni, and recently again observed by M. Jamin.

Lastly, it is well known that mechanical blows aid the assumption of magnetic power in steel, but tend to lessen and can even destroy it when assumed; and the same also is true of heat, which no doubt acts in a similar way, viz. by lessening the cohesion of the particles of steel.

All these facts may be embraced under the assumption first made by M. De la Rive, that magnetization is expressed by a definite movement, or a marshalling of the molecules of ironthe placing, as Dr. Tyndall puts it, of their longest dimensions end to end. Now iron is not the only magnetic body. Nickel and cobalt share the magnetic properties of iron to a very high degree; and to a much less extent the metals chromium and manganese are also magnetic. If, then, magnetization is an act associated with an altered structure of iron, we should expect to find a certain correspondence to iron in the properties of the other magnetic metals. That this is the case, is I think shown by the paper which has preceded this, "On the Relationship of the Magnetic Metals" *.

I was anxious to try further whether the molecular disturbances found on magnetizing iron were also exhibited by nickel and cobalt. One would of course expect to find analogous changes in these bodies; but I am unaware that they have hitherto been examined.

Messrs. Johnson and Matthey very kindly lent me an extremely fine bar of nickel and one of cobalt. Both bars are cylindrical, a little over 9 inches long and 1 inch in diameter. Though as pure specimens as they can be rendered commercially, the cobalt I find contains a very appreciable amount of iron; the removal of which body, as chemists well know, is a matter of the utmost difficulty.

The relative magnetic powers of these two bars deserves a moment's consideration. Nickel is invariably ranked above. cobalt in the scale of magnetic metals, Faraday and others placing it next to soft iron. But the bar of nickel I have used, when submitted to the same magnetizing current as the cobalt bar, exhibits far less portative force than the cobalt. It is remarkable that the iron impurity contained in the cobalt is able to produce so powerful an influence. The nickel, like other speci* Phil. Mag. Dec. 1873.

mens I have met with, has very slight retentive power when magnetized, whereas the cobalt has a high degree of coercive force.


Enclosing either of the bars within a helix of wire, a sound was emitted as soon as an interrupted current traversed the helix. The sound with cobalt was far the more powerful of the two, and was even more pronounced and more metallic than with a corresponding bar of iron. This fact, I believe, has not been noticed before. It is easy to obtain these sounds by merely using the coil of an electromagnet and drawing the terminal of the battery wire over a coarse file in a distant room.


In order to examine whether the metals lengthened by magnetization, I had a special apparatus made for me by Messrs. Yeates, of Dublin and London. The instrument is a modification of the arrangement used by Dr. Tyndall, and described by him in his 'Researches on Diamagnetism' (p. 240). Instead of being mounted vertically, the iron bar in my instrument is placed horizontally within the coils of a powerful electromagnet. One end of the bar is rigidly pressed by the end of a micrometer-screw, which is mounted on a sliding brass support that can be adjusted to any length that the bar under experiment may be. The other end of the bar presses against a system of levers, by which the least motion of the bar is largely multiplied. On an axle moved by the last lever a mirror is fixed; and upon this a beam of light is thrown, the reflected image being received on a distant scale. The whole is mounted on a firm mahogany base rather more than a metre long. The momentary warmth of a lighted match against the iron bar drives the reflected image through 2 or 3 feet of the scale.

The arrangement is shown in fig. 1.

Fig. 1.

The soft iron bar, II,

[blocks in formation]
« AnteriorContinuar »