XVII. THE POLYMAGNET.*

THE polymagnet was devised for the purpose of exhibiting before a class of pupils as many as possible of the phænomena of electro-magnetism and diamagnetism.

The instrument consists of an arrangement of two horseshoe electro-magnets, a helix of covered copper wire disposed between them, and suitable means of suspension.

A section of one of the electro-magnets, and its surrounding spirals is given, fig. 1, Plate IV. ab, cd are two cylindrical cores of soft iron, which are united by a cross-piece of the same material, ef. Through the cross-piece pass the strong screws g and h into the cores, and by them the ends b and d of the cores, which are accurately planed so as to ensure perfect contact with the cross-piece, are attached to the latter. The diameter of the cores is 1.125 inch, and their distance apart, from centre to centre, 4.85 inches; the cross-piece ef is drawn in proportion.

Round each core is a helix of copper wire, overspun with cotton, which was afterwards saturated with shell-lac. In winding the helix, two lengths of wire, one covered with red cotton and the other with green, were laid side by side and coiled as a single wire. The diameter of the wire is 0.1 of an inch, and the weight of it which surrounds each limb of the magnet is 12 lbs. For all four limbs, therefore, a weight of 48 lbs. is made use of.

The second electro-magnet is in every respect similar to the one just described.

Fig. 2 is a front view of a helix of covered copper wire, intended to be placed between the two electro-magnets; it has an internal diameter, ab, of 1 inch; an external diameter, cd, of 8 inches, and measures along its axis 1.15 inch. The diameter of its wire is 0.065 of an inch, and its weight is 6 lbs. ; it is wound so as to form a double coil, as in the case of the electro-magnets. Phil. Mag., vol. ix. p. 425.

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The radial strips, and central and surrounding ring seen in the figure, are of brass, and hold the coils of the helix compactly together.

Fig. 3 represents a stout slab of mahogany which supports the apparatus. ab, cd are hollows cut in the slab to receive the cross-pieces of the two electro-magnets; from e to ƒ the slab is cut quite through, the cross-pieces merely resting on the portions between ƒ and b, f and d, &c. The small apertures at x x show where the screws enter which attach the cross-piece to the slab of wood. The central aperture at g shows where the pin g' of the helix, fig. 2, enters, the helix thus occupying the central portion of the board. Right and left are two projections for the reception of two current reversers, which will be described immediately. The apertures 1, 2, 3, 4 are for the reception of pins projecting from a glass case intended to cover the whole apparatus.

When the magnets and central helix are fixed in their places and looked down upon, their appearance is that represented in fig. 4; at a and c the tops of the cores are seen, the movable masses of soft iron which belong to them being removed; the two ends of the other electro-magnet bear two such masses, each formed from a parallelopiped 4.5 inches long, 2 inches wide, and 1.25 inch high, having one end bevelled off so as to render it pointed, the other end being suffered to remain flat. The distance between those movable masses may be varied, or the body to be examined may be suspended either between surfaces or points, according to the nature of the experiment. The projections of the current reversers upon the horizontal plane are seen right and left.

Simplicity and efficiency being the objects aimed at, a current reverser was devised which fulfils these conditions. A front view of the instrument is given in fig. 5, and its horizontal projection in fig. 6. Q is the section of a quadrant of wood, which is capable of being turned by the handle HP; ab is the section of a strip of brass laid on the periphery of the quadrant; cd is a shorter strip similarly laid on; between b and c is a gap, formed of the wood of the quadrant itself, or of a piece of ivory or glass inlaid; s and s' are two brass springs,* which are shown in the figure to rest upon the strips of brass ab and cd; M M' are two

*Copper, I think, would be better than brass.

clamps secured to the wooden pillars O and O' by screws S, which pass up through the latter. The plan below corresponds to the section above. From b, fig. 6, the strip of brass crosses to c', and from c to b', both being insulated from each other at R. Supposing, then, the two clamps M and L to be connected with the two poles of a galvanic battery, the current entering at M would flow along the spring s to b, thence to c', and finally along the spring s to the clamp L': in like manner the current entering at L would attain the clamp M'. In this position of things the handle of the instrument leans to the left, as in fig. 5. If the current is to be interrupted, this is secured by setting the handle vertical; for when the handle is in this position, the spring s' rests upon the non-conducting surface bc, and the circuit is broken. If it be desired to send the current direct from L to L', and from M to M', this is accomplished by causing the handle to lean to the right; when this is done, both the springs ss' rest upon the self-same strip of brass ab, and there is direct metallic communication between L and L', and between M and M'. This reverser has been tested practically, and found very convenient. It is very similar to an instrument devised by Professor Reusch, but simpler and more easily constructed.

Fig. 7 is a plan of the top of the glass case which surrounds the polymagnet. At C a brass tube is cemented to the glass, which is here perforated, and through the tube a rod passes furnished at its lower extremity with a hook, to which is attached a suspending fibre. ab is a horizontal brass cylinder capable of being turned on its axis by the milled head at a, and thus coiling a suspending fibre around a groove marked at d: the cylinder is also capable of sliding right and left, so that the body suspended from the fibre may be moved laterally, and the amount of motion measured on the graduated bar above.* a'b' is another horizontal suspension rod, in every respect similar to the former.

The whole instrument, surrounded by its glass case, is shown in perspective in fig. 8. The magnets are visible, with the moveable masses of soft iron resting upon them; in the centre is seen the helix sketched in fig. 2, and within the helix a bismuth bar supported by several fibres of unspun silk attached to the central

*This arrangement, though very convenient for private research, is not necessary for lecture experiments.

rod which passes through the top of the glass case. The manner of suspension of the bismuth will be understood from the drawing, certain practical artifices which suggest themselves when the drawing is attentively inspected being introduced to facilitate the placing of the axis of the bar along the axis of the surrounding helix. The current reversers are seen without the case; two opposite sides of the latter can be opened by the handles h and h', so that free and easy access to the interior is always secured.

Experiments to be made with the Polymagnet.

1. All the experiments that are usually made with an upright electro-magnet.

2. The various portions of the instrument may with great facility be lifted separately out of the case. One of the electromagnets being thus removed, a rope can be passed through a ring r, introduced for this purpose into the cross-piece, fig. 1: adjacent to the screws g and h two plates of brass are seen; these are attached to the brass reels of the helices, and by passing a pin through the holes shown in the figure, the helices are prevented from falling when the magnet is turned upside down. Attaching the rope to a hook in the ceiling, or to a strong frame made for the purpose, experiments on the lifting power of the magnet may be made.

3. While one of the magnets is suspended as last described, the other, which is of exactly the same size, can be brought up against it, the free ends of the four cores being thus in contact. The same currents being sent through both magnets, we have the mutual attraction of two electro-magnets instead of the attraction of an electro-magnet for a mass of soft iron, as supposed in the last experiment. The arrangement just described is indeed precisely that devised by M. Pouillet in the construction of a powerful electro-magnet for the Faculty of Sciences at Paris. To the cross-piece of the second magnet a ring is also attached, from which weights can be suspended.

4. The cross-pieces may be removed by withdrawing the screws g and h, and the spirals may be made use of singly with their corresponding bar-magnets. As two wires surround each coil, one of them may be used to exhibit the induced currents developed by the other. The phænomena of the extra-current