DESCRIPTION OF SOME TYPES OF ELECTROMAGNETS 257

the preference. The investigations of Joule on the lifting power of magnetic circuits (§§ 105, 111) led him to the construction of very powerful electromagnets, which are even now met with, and are known as Joule electromagnets' (fig. 50). They are distinguished by their length, and may be constructed from

a stout iron tube cut lengthwise-for instance, a gun barrel. Instead of the horseshoe shape, electromagnets are now much used in which the coils are on straight limbs connected through a rectangular iron yoke, the keeper being usually of the same shape.

With these are connected what are called clubfooted' electromagnets, in which one limb is coiled, while the other is useful only in closing the magnetic circuit; the latter, according to the plan of Nickles, may also be effected by two or more magnetomotively 'dead' iron bars, which are symmetrically arranged about the coil, and the total section of which is at least equal to that of the core inside. These various bars may, lastly, also be merged into a cylindrical iron shell; this leads us to the type of the iron-clad Bell Magnets' introduced by Guillemin and Romershausen. The iron shell is connected with the core by means of an iron disc, the keeper being of the same form. Similar electromagnets, with several iron shells and coils fitted into each other, have been constructed by Camacho, but hardly appear to offer theoretical advantages.

The ironclad forms represent a transition to the very effective electromagnets in which the conductor is embedded in

The keeper is not usually covered with wire; but from the considerations in § 94 and § 95, it scarcely matters where the requisite number of ampereturns is placed, that is where the magnetomotive force chiefly originates.

S

narrow grooves in the ferromagnetic substance itself. To these belong the older electromagnets of Roberts and Joule, the construction of which is sufficiently obvious from figs. 51 and 52. An arrangement described by Forbes and Timmis may finally

be mentioned, which is represented in fig. 53, and may be regarded as a modification of Radford's, in which the windings are spiral instead of circular, as in the former. The electromagnets last mentioned have been chiefly constructed with a view of trans

FIG. 53

mitting motion by means of the friction set up, and also for brakes on carriage wheels as represented in fig. 53. It may be mentioned in this connection that what is called magnetic

friction' is essentially due to the greater pressure between the surfaces in contact. The question whether and in how far the coefficient of friction itself is affected by magnetisation, possibly in consequence of molecular processes on the surface, must for the present be considered as an open one.'

C. Electromagnets for producing Strong Fields

§ 167. Review of the usual Types. The problem of producing strong magnetic fields is of great interest for experimental physics; the investigation of many phenomena, especially those in which the square of the intensity of the field comes into play (§§ 203, 229), can only be successfully carried further by using

For further details we may refer to the work of Silv. Thompson. See also G. Wiedemann, loc. cit. §§ 358-371.

RUHMKORFF'S ELECTROMAGNET

259

the strongest possible fields. The question as to the rational design of electromagnets for this purpose has, nevertheless, scarcely been considered. We shall discuss this question, therefore, somewhat more in detail than the electromagnetic arrangements hitherto mentioned, and the more so as we shall derive from it an interesting application, as well as a confirmation of the theory of Chapter V. (§§ 171-173).

Of the types of construction which have been empirically arrived at, perhaps those in most extensive use are copied from horseshoe magnets; like these (§ 166), they consist of two vertical coiled limbs, the lower ends of which rest on a massive iron yoke, while to the upper ends pole-pieces of suitable form

may be fitted. With vertical electromagnets the legs are usually very long, in order to bring in the requisite number of turns; owing to their being so near one another there is considerable leakage between them, which diminishes the available flux of induction between the pole-pieces.

The electromagnets of Ruhmkorff's construction are widely known, and are as effective as they are convenient to use (fig. 51). The two angular iron pieces 0 0 and O'O' may be moved horizontally, and may be clamped in any given position, by which the space between the pole-pieces may be conveniently adjusted; to these may be screwed the horizontal cores, which for magnetooptical experiments are bored. The position of the coils M and M' is in any case far more rational than in the vertical

electromagnets (§ 173). The objection may be raised that the circuit 0 KO' is too weak magnetically as well as mechanically; the consequence of this is that, on the one hand, its magnetic reluctance is greater than necessary, and, on the other, the two angle pieces bend under the influence of magnetic tractive force, by which the distance of the poles is diminished. The magnetomotive force might moreover be considerably increased by coiling the lower connecting piece K.1

In this respect, the magnetic circuit of the electromagnet represented in fig. 55, and used by Ewing and Low in their

isthmus method (§ 217), is better arranged. It is true that this is at the cost of convenience of manipulation, which in Ruhmkorff's construction is unexcelled; this drawback might, however, be remedied by suitable mechanical arrangements. The electromagnet represented could be excited with 61,000 ampere-turns;

[These objections might be met by suitably designing connecting pieces of greater cross-section and rigidity; the adoption of modern cast steel' of high permeability would probably render such a type the best and cheapest for producing all but the very strongest fields.-H. du Bois ]

PRINCIPLES OF DESIGN

in this way Ewing and Low obtained an intensity

261

= 24,500,

and an induction B = 45,350 C.G.S., the highest which had at that time been obtained in soft iron. As regards the field intensities observed in the air, it follows from the published statements that until recently values above 28,000 to 30,000 C.G.S. units had not been reached.

§ 168. Principles of Design.-There seemed no a priori reason why the production of still stronger fields should be impossible. The author accordingly attempted the construction of an electromagnet for this purpose, and in this attempt he was guided by the following considerations. The first point is to begin with the production of as high a value as possible of the flux of induction, which then, by 'throttling' the magnetic circuit by means of suitable pole-pieces, may, as it were, be concentrated as described below (§ 175). Accordingly the magnetic reluctance which, especially owing to the unavoidable air-space between such pole-pieces, cannot be indefinitely diminished, must be overcome by as great a number of ampere-turns as possible (§ 173).

In all the electromagnetic apparatus and machines we have hitherto discussed, and indeed in the great majority of such, it was sufficient from the nature of the case to consider only the first two stages of the process of magnetisation. But in the present problem the third, or stage of saturation, alone need be considered. In consequence of this, and of the circumstance that considerations of economy, certainty of working, facility of repair, and the like, are of less account in the present case, the conditions of construction are, to some extent, different. The discussion of § 95 showed that the field of the coil finally tends to completely direct and dominate the distribution of the vectors in the magnetic circuit; hence the coiling must be such that there is everywhere, and especially between the pole-pieces, a field in the desired direction-that is, tangential to the centroid of the magnetic circuit. In such an arrangement leakage will ultimately decrease as the saturation increases, and the inductiontubes so gained will be utilised; this result was confirmed by experiment (§ 173). As regards the shape of the ferromagnetic substance, the theoretical conditions already mentioned are best satisfied by a toroid divided radially. In other respects, the points discussed in § 141 for the construction of the frames of