magnetise steel bars when they are passed through a copper wire coiled in a helix round the bars.

The intensity of the shock produced by induced currents renders their effects comparable to those of electricity in a state of tension. But as they act on the galvanometer the electricity is present, both in a state of tension and in the dynamical condition.

These phenomena of induction currents are well seen in Ruhmkorff's coil, which we shall now describe.

481. Ruhmkorff's coll.-This is an arrangement for producing induced currents, in which a current is induced by the action of an electric current, whose circuit is alternately opened and closed in rapid succession. These instruments, known as inductoriums, or induction coils, present considerable variety in their construction, but all consist essentially of a hollow cylinder in which is a bar of soft iron, or bundle of iron wires, with two helices coiled round it, one connected with the poles of a battery, the current of which is alternately opened and closed by a self-acting arrangement, and the other serving for the development of the induced current. By means of these apparatus, with a current of three or four Grove's cells, physical, chemical, and physiological effects are produced equal to and superior to those obtainable with electrical machines and even the most powerful Leyden batteries.

Of all the orms those constructed by Ruhmkorff in Paris, and by Ladd and Apps in this country, are the most powerful. Fig. 396 is a representation of one, the coil of which is about 14 inches in length. The primary or inducing wire is of copper, and is about 2 mm in diameter, and 4 or 5 yards in length. It is coiled directly

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on a cylinder of cardboard, which forms the nucleus of the apparatus, and is enclosed in an insulating cylinder of glass, or of caoutchouc. On these is coiled the secondary or induced wire, which is also of copper, and is about mm. in diameter. A great point in these apparatus is the insulation. The wires are not merely insulated by being in the first case covered with silk, but each individual coil is separated from the rest by a layer of melted shellac. The length of the secondary wire varies greatly; in some of the largest sizes it is as much as 60 miles. With these great lengths the wire is thinner, about mm.

The following is the working of the apparatus. The current arriving by the wire, P, at a binding screw, a, passes thence into the commutator, C (fig. 398); thence by the binding screw, b, it enters the primary wire, where it acts inductively on the secondary wire ; having traversed the primary wire it emerges by the wire, s. Following the direction of the

wire only acts inductively on the secondary wire (478), when it opens or closes, and hence it must be constantly interrupted. This is effected by means of the oscillating hammer, o, omitted in figure 397, but represented on a larger scale in fig. 398. In the centre of the bobbin is a bundle of soft iron wires, forming together a cylinder a little larger than the bobbin, and thus projecting at the end as seen at A. When the current passes in the primary wire, this hammer, o, is attracted; but immediately, there being no contact I between o and h, the current is broken, the magnetisation ceases, and the hammer falls; the current again passing, the same series of phenomena recommences, so that the hammer oscillates with great rapidity.

In proportion as the current passes thus intermittently in the primary wire of the bobbin, at each interruption an induced current, alternately direct and inverse, is produced in the secondary wire. But as this is perfectly insulated, the current acquires such an intensity as to produce very powerful effects. Fizeau has increased this intensity by interposing a condenser in the induced circuit. As constructed by Ruhmkorff, for his largest apparatus, this consists of 150 sheets of tinfoil about 18 inches square; these sheets being joined are coiled on two sides of a sheet of oiled silk, which insulates them, forming thus two armatures; they are then coiled several times round each other, so that the whole can be placed below the helix in the base of the apparatus. One of these armatures, the positive, is connected with the binding screw, i, which receives the current on emerging from the bobbin ; and the other, the negative, is connected with the binding screw, m, which communicates by the plate, K, with the commutator, C, and with the battery.

482. Effects produced by Ruhmkorff's coil.—The high degree of tension which the electricity of induction coil machines possesses has long been known, and many luminous and calorific effects have been obtained by their means. But it is only since the improvements which Ruhmkorff has introduced into his coil, that it has been possible to utilise all the tension of induced currents, and to show that these currents possess the properties of statical as well as dynamical electricity.

Induced currents are produced in the coil at each opening and breaking of contact. But these currents are not equal either in duration or in tension. The direct current, or that on opening, is of shorter duration, but more tension; that of closing of longer duration but less tension. Hence if the two ends P, and P', of the fine wire (figs. 397 and 398) are connected, as there are two equal and contrary quantities of electricity in the wire the two currents neutralise each other. If a galvanometer is placed in the circuit, only a very feeble deflection is produced in the direction of the direct current. This is not the case if the two extremities, P and P', of the wire are separated. As the resistance of the air is then opposed to the passage of the currents, that which has most tension, that is, the direct one, passes in excess, and the more so the greater the distance of P and P' up to a certain limit at which neither pass. There are then at P and P' nothing but tensions alternately in contrary directions.

The effects of the coil, like those of the battery, may be classed

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under the heads physiological, chemical, calorific, luminous, mechanical; they have this difference, that they are enormously more intense.

The physiological effects of Ruhmkorff's coil are very powerful ; in fact, the shocks are so violent that many experimenters have been suddenly prostrated by them. A rabbit may be killed with an induction current arising from two of Bunsen's elements, and a somewhat larger number of couples would kill a man.

The calorific effects are also easily observed; it is simply necessary to interpose a very fine iron wire between the two ends, P and P', of the induced wire; this iron wire is immediately melted, and burns with a bright light. The spark of the Ruhmkorff's coil is used to fire mines in military and mining operations.

The chemical effects are very varied, inasmuch as the apparatus produces both dynamical electricity and electricity in a high state of tension. Thus, according to the shape and distance of the platinum electrodes immersed in water, and to the degree of acidulation of the water, either luminous effects may be produced in water without decomposition, or the water may be decomposed and the mixed gases disengaged at the two poles, or the decomposition may take place, and the mixed gases separate either at a single pole or at both poles.

The luminous effects of Ruhmkorff's coil are also very remarkable, and vary according as they take place in air, in vacuo, or in very rarefied vapours. In air the coil produces a very bright loud spark, which, with the largest-sized coils, has a length of eighteen inches. In vacuo the effects are also remarkable. The experiment is made by connecting the two wires of the coil, P and P', with the two rods of the electrical egg (fig. 328), used for producing in vacuo the luminous effects of the electrical machine. A vacuum having been produced, a beautiful luminous trail is produced from one knob to the other, which is virtually constant, and has the same intensity as that obtained with a powerful electrical machine when the plate is turned.

If this light be closely observed, it will be found that if some vapour of turpentine, or wood spirit, or bisulphide of carbon, have been introduced into the globe before exhaustion, instead of being continuous, the light consists of a series of alternately dark and bright zones, forming a pile of electric light between the two poles. This phenomenon is known as the stratification of the

electric light, and is due to the circumstance that the current is discontinuous.

The brilliancy and beauty of the stratification of the electric light are most remarkable when the discharge of the Ruhmkorff's coil takes place in glass tubes containing a highly rarefied vapour or gas. These phenomena, which have been investigated by Masson, Grove, Gassiot, Plücker, etc., are produced by means of sealed glass tubes first constructed by Geissler, of Bonn. These tubes are filled with different gases or vapours, and are then exhausted. At the ends of the tubes two platinum wires are soldered into the glass.

When the two platinum wires are connected with the ends of a

Ruhmkorff's coil, magnificent lustrous striæ, separated by dark bands, are produced all through the tube. These striæ vary in shape, colour, and lustre with the degree of the vacuum, the nature of the gas or vapour, and the dimensions of the tube. The phenomenon has occasionally a still more brilliant aspect from the fluorescence which the electric discharge excites in the glass.

Fig. 399 represents the striæ given by hydrogen; in the bulbs the light is white, in the capillary parts it is red.

In carbonic acid the colour is greenish, and the striæ have not the same shape as in hydrogen; in nitrogen the light is greenish yellow.

Mechanical effects. Ruhmkorff's coil also produces mechanical effects so powerful that, with the largest apparatus, glass plates two inches thick have been perforated. The result, however, is not obtained by a single charge, but by several successive charges.

The experiment is arranged as shown in fig. 400. The two poles of the induced current correspond to the binding screws, a and b ; by means of a copper wire, the pole, a, is connected with the lower part of an apparatus for piercing glass like that already described