the lines to, and concentrating them at almost any point we please. It may be mentioned that the relative facility with which the lines of force are propagated by good soft iron, or the electromagnetic conductivity of the iron as compared with that of air, may under exceptional circumstances be as high as 2,000 to 1. If, then, we place a bar of iron inside a solenoid, a large percentage of the lines of force lose their circular form, and, passing through this iron 'core' (as it is called), they leave it at its ends to complete their excursion round the wire from which they were generated. An example of this action of iron is shown in fig. 41,' which illustrates the field developed by a powerful current travelling through a solenoid of a few turns, having a core of comparatively small dimensions. Were a larger and more massive core to be introduced, even more of the lines of force would extend We are indebted to the proprietors of Engineering for permission to reproduce figs. 41 to 44. themselves through the iron instead of circulating in the immediate vicinity of the wire. By the introduction of these iron cores the strength of the action between two solenoids is enormously increased, and a still further increase can be obtained by so shaping the iron that the greatest possible facilities are offered for the lines of force set up by the one solenoid to traverse the space occupied by the majority of those due to the other. The designing of the shape and dimensions of an iron core often becomes an extremely important matter, as, for example, in the case of the dynamo electric machines hereafter to be described, in which it is necessary to concentrate an exceedingly powerful field in such a manner that it shall be approximately uniform over a comparatively large space. In such cases the iron cores may be more than a ton in weight, and they are not only expensive to construct, but a considerable expenditure of energy is also required to keep them magnetised. There is, therefore, great scope for effecting economy by making the design such that the cores may be cheaply constructed and yet act efficiently. The student will do well in such cases to always endeavour to think of the iron as simply affording a means of diverting the lines of force set up by the current into just that part of the field where they are required to act. The precise effect on the iron itself is, to a great extent, still a matter for speculation, but it must be remembered that the only way of actually increasing the number of the lines of force setting up the field is by either increasing the current itself, or by increasing the length of the wire and adding to the electro-motive force sufficiently to maintain the same current strength. The amount of attraction or repulsion exhibited by the solenoids furnished with their iron cores might be used as a means of measuring current strength, but the arrangement is not a convenient one, owing chiefly to the difficulty in obtaining perfect freedom of motion, the liability of variation in the current strength, and the varying properties of the iron. Were it not for these disadvantages we could keep the electromagnetic force of one of the solenoids constant, and send the currents to be compared and measured through the other. But here Nature comes in to aid us, for it is found that if a piece of hard iron or steel is used as a core for the solenoid, it retains more or less permanently a large portion of the electro-magnetic properties originally produced by the current. The power or ability of retaining such effects or properties is known as the ' retentivity' of the iron or steel, and depends entirely upon its chemical composition and mechanical or molecular structure. This retentivity is the same property as that hitherto known as 'coercive force,' which was certainly a misnomer. If retentivity corresponds to anything at all, it is to inertia rather than to anything that can fairly be described as coercive. The similarity to mechanical inertia is seen in the fact that those samples which transmit the lines of force freely, retain scarcely any of the influences producible by them, while, on the other hand, hard iron and steel, which resist the propagation of the lines of force, resist FIG. 42. equally well the vanishing of these lines after the cessation of the current which called them into existence. A piece of iron or steel which acquires and so retains the power of acting as a solenoid, is called a permanent magnet, or simply a magnet, and its extremities are also called poles. The lines of force still enter and leave the steel as they did before it was removed from the helix, the direction being shown in fig 42. The end, s, at which they enter is called the south-seeking pole of the magnet, and the other its north-seeking pole. The actual arrangement imparted to iron filings sprinkled on a sheet of paper placed over a permanent steel magnet is beautifully illustrated in fig. 43. Such a distribution of the filings would take place in any plane parallel to the axis of the magnet, for the lines of force radiate from the poles similarly in all directions. The distribution observed when the paper is placed on the end of the magnet and at right angles to its axis is shown in fig. 44. If we know the direction in which the current passes round a piece of iron or steel, it is easy to predict the direction of its polarity; for if we look at the end of the bar, and the current is then flowing round it in a right-handed direction, as in fig. 45, that end will be a south-seeking pole; but if the current flow in a left-handed direction, as in fig. 46, that end will be a northseeking pole. It is the practice to enter into a detailed description of the difference between right- and left-handed helices with a view to facilitating a recollection of the electro magnetic polarity. Thus a left-handed helix (fig. 47) is one in which, from whichever end the current enters, it will travel in the opposite direction to that taken by the hands of a clock, and will develop north-seeking polarity at the end at which it enters and south-seeking polarity at the end of emergence. Conversely, a right-handed helix (fig. 48) is one in which the current will travel round in the same |