ful bar magnet of rectangular cross-section, the end of the magnet and the end of the bobbin which are turned towards one another being of the same polarity. Fig. 76 shows how the lines of force issuing from the interior of the bobbin W W are deflected on encountering the lines. which emerge from N, bending round this pole as they might round an impenetrable obstacle. Like the stream lines from two

FIG. 76

neighbouring sources which send forth fluid against one another, the lines of force emanating from the helix produce a bending in the lines which proceed from N, and which ultimately pass round to the south pole (outside the figure), beyond the indifferent zone i of the magnet. Thus it is that there is an indifferent point or zero point J between the end of the bobbin and the pole of the magnet, the field-intensity being inappreciable through a small region surrounding this point. If the magnet is movable, it will be driven away axially from the pole of the bobbin, owing to the pressure across the direction of the lines of force. On the other hand, when the magnet is fixed, the bobbin, if movable, will be thrust in the opposite direction, that is, away from the magnet.

If the magnet pole of opposite name be turned towards the end W W of the helix, the phenomena will be reversed, the line-of-force diagram being similar to fig. 77. The lines of force emerging from the bobbin stream into the south pole (sink) of the magnet. The tension along the lines of force tends to drag the magnet into the interior of the bobbin, or, when the magnet is held fixed, to drag the bobbin over it like an enclosing case.

The mechanical actions of bobbins upon coaxally placed bar magnets were made the basis of many of the older electro

motors. When the current through the bobbin is made to alternate in direction, the magnet is correspondingly drawn in and thrust out, so that a reciprocating motion results, like that of the piston of a steam engine.

(b) Action of a bobbin on soft iron.-Since iron collects together the lines of force, that portion of the iron bar which is turned towards the bobbin becomes a sink (south pole); hence the iron is drawn towards those places where the lines of force are densest, that is, from the external field into the interior of the helix, and this independently of the direction of the current. Herein lies the wide applicability of the phenomenon, for example, in the regulation of arc lamps.

In the channel E of the apparatus, fig. 74, is laid a short piece of soft iron. If now a current be made to circulate in the bobbin in either direction, the line-of-force diagram obtained will be like fig. 77. The lines of force emerging from the interior of the helix, between the wind

ings W W, do not diverge equally in all directions as in fig. 75, but pass for the most part to the soft iron Fe of greater permeability, entering it at e and making their way through its length. At the further end a they once more emerge into the field. In the neighbourhood of a the lines of force are much denser than if no iron were present. The end e has become a sink,' and the end a a new source.

The remaining lines of force, which have not passed through the iron, are spread out less widely than they would be in the undisturbed field. The iron has collected together the lines of force to such an extent that the field from h to e in the figure is nearly uniform. If the iron rod Fe is free to move, it will be drawn into the interior of the helix owing to the tension along the lines of force which terminate at e.

The strength of the pull exerted upon iron depends upon the number of lines of force concerned. This number is approximately proportional to the current-strength, for the measurement of which the phenomenon may accordingly be employed.

Experiment 60.-A helix wound with many turns of wire is supported vertically upon three feet, and an iron core which can pass easily through the helix is supported below with its upper end just within the windings. When a strong current is passed through the helix, the iron core is lifted up and sucked into the interior of the coil; on breaking the circuit it falls down again.

182. Spring ampère-meter.-A bobbin of wire is fastened in a vertical position upon a stand, or against a wall, and a long vertical guiding rod is fixed so as to be coaxal with the bobbin. Sliding upon this rod within the bobbin is a thin cylindrically bent piece of sheet iron, which is attached to a helical spring above. When there is no current flowing, only the lower end of the sheet-iron cylinder dips into the interior of the bobbin, but when the circuit of the current is closed, the cylinder is drawn down into the bobbin against the resistance of the spring, the displacement being the greater the stronger the current employed. The value of the current in ampères is given by an index attached to the iron cylinder and moving over a vertical scale; this latter having previously been empirically graduated so as to give direct readings' (F. KOHLRAUSCH).

In lectures and demonstrations it is very convenient always to be able to show the value in ampères of the currents used in an experiment; in this way not only is the meaning of the term kept in view, but a definite conception becomes associated with a

definite number of these practical units. It is therefore very desirable to be able to show to a large audience a measuring instrument of considerable size and of a type commonly employed, especially in technology. KOHLRAUSCH's spring ampère-meter is convenient for the purpose, as without much expense it can be made of such dimensions that its indications can be read from a considerable distance.

183. Uniform field within a helix.--A bobbin traversed by a current provides one of the most important ways of producing a strong uniform magnetic field. If the bobbin is sufficiently long, the effects of the free ends soon become negligible as we pass to the interior, where the lines of force run almost exactly parallel to one another. We shall make use of the uniform field in the interior of a helix to render evident the high permeability which is characteristic of soft iron.

Once more place the short iron bar in the channel E of the apparatus, fig. 74, and fasten to it with sealing-wax a strip of cartridge paper whose breadth is equal to the bore of the bobbin. After sprinkling the paper uniformly with iron dust, push it, along with the iron bar, into the interior of bobbin. On closing the circuit of the current for a short time, and tapping the apparatus, we obtain a line-of-force diagram such as fig. 78.

FIG. 78

The lines of force run parallel to one another in the interior of the helix, until they approach the extremity of the iron rod Fe, towards which they strongly converge. They pass through the iron, and emerge again at the further end, where they once more spread out; but at greater distances from the iron they become more and more parallel to one another, so that the field regains its uniform character. Lines of force crowd into the iron from the surrounding field. Soft iron in the interior of a helix traversed by a current collects together the lines of force.

Since the lines of force which enter Fe at one end are exactly the same in number as those which leave it at the other end, and since the lines of force at a little distance neither converge nor diverge, the mechanical forces exerted upon the iron will just neutralise one another's effect, so that there is on the whole no force tending to move the iron in one direction or the other.

The directing influence exerted upon iron particles by the uniform field in the interior of a helix may be rendered evident to a large audience by projection with a lantern as follows: A short wide glass cylinder is provided with two glass discs which can be screwed firmly against its ends so as to close them (as in apparatus for investigating the rotation the plane of polarisation by a sugar solution), and upon this cylinder several layers of thick insulated copper wire are wound. The cylinder is filled with a mixture of glycerine and iron filings, and is placed in front of the lantern, in the axis of the emergent beam of light. The picture of the cross-section of the cylinder, when formed upon the screen, appears dark, because the light is cut off by the irregularly distributed iron filings. If, however, a strong current is sent through the helix, the particles arrange themselves in axial chains, and allow light to pass.

184. Magnetising helix.--The strong uniform magnetic field which exists in the interior of a helix through which a current is flowing is used to magnetise steel bars. This is a far more effective method of magnetisation than that of rubbing with another magnet, as described in § 19, for in the present case we make use of far denser bundles of lines of force, and can therefore cause a much greater number of lines to pass through the steel, even when the coercive force (§ 18) is greater. If the helix is longer than the steel rod to be magnetised, the magnetic force due to the current is everywhere the same along the length of the rod, which thus becomes very uniformly magnetised.

Experiment 61.-Let unmagnetised steel knitting-needles be placed within a long helix through which a current is flowing. On stopping the current and removing the needles, the latter will be found to act as bar magnets of considerable strength.