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a good many of the little bars may be removed from the spiral without weakening the induced current; this was particularly the case when the condenser was introduced.

But although the horseshoe form of the electro-magnet above mentioned did not produce the desired effect, one advantage of this arrangement remains valuable, that is, the use of the interruptor.

When a strong iron or steel spring is fastened to one pole, and reaches closely over the other, terminating there in a piece of iron of the shape of a hammer, this serves as a continuation of the magnet, and will be very strongly attracted by the other pole. Nearly in the middle of the spring is placed a piece of brass coated with platinum; above the latter, upon a stand, is the regulating screw with its platinum point, which carries the current from the battery to the spring and to the commencement of the primary coil. The vibrations of this strong spring are very energetic: they effect a sure closing and an exact interruption of the circuit.

Upon each limb of the horseshoe magnet, the wire of which had been previously protected by a coating of strong gutta-percha, stands an induction coil, the body of which, consisting of wood, was also coated with gutta-percha previous to the winding of the wire. The interior end of the fine wire is carried outwards through a glass tube. Each coil has an interior diameter of 45 millims. The windings reach to a height of 120 millims.; the exterior diameter of the coil is 85 millims. As the horseshoe magnet stands vertical, both ends of the spirals are carried upwards through the wood of the spiral, and terminate in insulated binding screws, which may be conveniently combined both together or in succession. By an arrangement for discharging, the distance which the sparks are to pass over can be altered at pleasure.

The whole of the phænomena hitherto known may be seen at the discharger; as, however, I did not succeed until afterwards in the construction of a good condenser, the striking distance of the sparks in air was not very considerable, although the appearances in rarefied air were very beautiful, and accompanied by all the peculiarities already so often described.

For the experiments in vacuo I did not use the so-called electric egg, that is, the ellipsoidal receiver, but a glass jar, 15 inches

, high and 5 inches in diameter, which could be closed above with a stopper, and conveniently placed on the plate of the air-pump; it was more easily cleaned than the egg. Besides this, in order to make the whole apparatus more portable, I have constructed for the experiments a suitable hand air-pump, which may be exhausted to at least 1 millim. barometric pressure, and which is also useful for other purposes. I shall take the liberty of describing it more accurately at a future time, and to explain it by drawings, as it is deserving of strong recommendation.

One of the phænomena in rarefied air which I had not hitherto' observed, and which I have nowhere found mentioned, is the following. It is known that by different means a stratification of the passing light may be obtained; this occurs most readily in the vapour of oil of turpentine or phosphorus, which, however, must be present in very small quantity. Whenever I obtained these layers, which appeared sharply defined, I observed a motion of them from one pole to the other. It is remarkable that the direction of this motion was immediately reversed when the two wires were changed so as to change the polarity of the two spheres. The progressive motion of the layers always takes place from the positive to the negative pole.

Although during the observation of these luminous layers, which, it may be remarked, are often so sharply defined that the intervening spaces are completely dark, an optical illusion may easily be thought of, possibly connected in some way with the play of the circuit breaker, I must mention that neither the magnitude of the dark spaces, nor the velocity of the motion was changed, when the breaker was permitted to vibrate at a different rate. If such layers once occur, the circuit may even be caused to open and close at pleasure by the hand; the same layers and the same progression of the strata will always be observed.

I now pass to the description of a new induction instrument, in the construction of which I have availed myself of my own experience and that of others, in order to produce the greatest possible effect.

The apparatus (fig. 1) consists of the following principal parts :-1, the primary coil with a bundle of iron wires; 2, the three single induction coils; 3, the interruptor ; 4, the discharger ; 5, the condenser.

1. The primary coil itself is not to be seen in the figure, but the upper end of the bundle of iron wires is seen at A. The latter consists of wires of 1 millim. in diameter, which stand loosely in a thin cylinder of wood, so that they may be taken out of it singly. The little bars are varnished with shell-lac. The primary coil is wound on the wooden cylinder; its wire is 1 millim. thick, and is formed of six layers of the double wire. The ends of the spiral are connected with the clamp screws B B. The height of the whole spiral is 20 millims. ; it has an external diameter of 50 millims., in which a coat of strong gutta-percha, 1 millim. thick, is reckoned. The wires are varnished for the sake of security.

2. The induction spiral consists of three single coils, which may be easily taken off the primary, and united

at pleasure one with another. The central cylinder of the coils consists of pieces

Fig. 1.

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of paper glued over each other and saturated with varnish. The walls of the coil are of wood, the best for this purpose being sound pear-tree wood; they are only 3 millims. thick, and have a diameter of 100 millims. In winding the fine wire and its insulation, the greatest care is known to be necessary. Although, by making three compartments, an approximation of the parts of the induction wire which lie at a great distance from each other is avoided, still the good spinning and good varnishing of the wire is by no means sufficient. It is easy to see that small sparks pass from one layer to that above it, when the excited spiral is observed in the dark. There is no means of avoiding this, except by adding to the varnished covering which surrounds the wires another coating which shall be impervious to such sparks. Fluid insulators indeed, such as oil of turpentine, close up the ruptured place immediately; but little is gained by this, as the passage of the sparks will not thereby be prevented. If one layer of wire be separated from the other by gutta-percha, wax-paper, &c., the sparks make a path for themselves round the edges and along the sides of the coil. Professor Poggendorff made use of a light fluid wax, which has been of greater service. The coils of the apparatus now in question are wound and insulated in the following manner :- A small glass tube is introduced at the inner

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side of one of the walls of the coil so deeply into the wood, that it does not project beyond the inner surface; at the circumference it protrudes a little. This tube receives a copper wire, to which the inner end of the fine wire is soldered. The winding in the lathe now commences. The wire is overspun with silk and varnished before it is laid on. After the completion of each layer, a mixture of 1 part of white wax and 1 part of colophonium is laid on hot with a camel's hair pencil, and the entire wire is kept warm by an alcohol lamp placed beneath it. By this means all spaces are penetrated and filled with this good insulating substance; the whole forms upon cooling a firmly connected cylinder, which it is hardly necessary to protect from external injury; for it is known that the mixture of wax and rosin is made use of as a very firm cement. The outer end of the connexions is also soldered to a strong copper wire, which is cemented in a small glass tube. Both ends of the wires are therefore well insulated by glass, and lie far from each other; the two copper wires project two lines outwards, so that binding screws may be readily attached to them.

All three coils are wound in the same manner and in the same direction; they are so pushed on to the interior spiral that the ends of all the interior wires stand on one side, while all the external ends are at the other, in a vertical direction. The manner of uniting the wires by intervening pieces will be manifest from the figure. The lowest coil rests upon three small pillars of ivory, which are not seen in the drawing. From the upper and under ends of the system of coils the wires are carried directly to the discharger.

3. The interruptor E is placed upon a wooden foot, and may also be made use of for other purposes ; as it can be taken away at once after the connecting wires have been removed. An electro-magnet, round which four layers of the same double wire which is used in the inner spiral are coiled, acts on a perforated and grooved iron cylinder F. The latter is attached to a strong lever, H, which is drawn downwards by a spiral spring. The platinum point of the screw H at the extreme end of the lever presses forcibly on the upper portion of a strong copper spring, J. On the upper surface of the spring is placed a disc covered with platinum, which can be caused to rotate, and is touched by the platinum point, not at the centre, but somewhere between the centre and circumference. This is for the purpose of changing at pleasure the point of contact without filing, which would be necessary if the platinum were fixed.

Under the projecting portion of the spring J there is a screw L, which, where it rises, supports the spring so that the latter

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loses its property of yielding, the descending lever thus meeting a firm surface below. Besides this, between the limbs of the spring a bit of cork is fixed for the purpose of damping the vibrations, which are not favourable to the development of the induced current. Finally, there is another screw K, by the turning of which a greater or less degree of tension may be imparted to the spiral spring. With this arrangement of the interruptor, we are able to fulfil, as completely as possible, the conditions which are necessary to the development of a powerful induced current.

When the making and breaking of the primary circuit is effected by the hand, by bringing the wires into contact, and then separating them quickly, one is soon convinced that the stronger the wires are pressed together before separation, and the more suddenly they are separated, the greater are the sparks which cross at the ends of the induction coil. It is just the same with the contact and separation of the platinum point and plate. The largest sparks are obtained when the spiral spring is in a state of the strongest tension, and the iron armature is not permitted to come too near to the electromagnet; for a portion of the force would be thus neutralized with which the lever is pressed by the spiral spring against the platinum plate.

Further, it is not indifferent whether the platinum point of the screw H meets at the closing of the circuit with a firm or a yielding surface; for in the first case the current will circulate only for a very short time, and the duration of the interruption will be greater than that of the closure of the circuit; in the latter case, on the contrary, where the spring J yields, we have

J a longer closure, and a time of interruption shorter or longer according to circumstances. By the use of the three screws H, L, and K, this relation can be altered at pleasure, and the play of the lever also permitted to proceed more slowly or quickly It is very interesting to observe the dependence of the induced current upon the manner of closing or breaking the primary current; and it is not to be wondered at, that in experiments where the separation has been effected between other metals, as silver, copper, gold, &c., such great differences should have been observed.

4. The discharger merely ministers to the convenience of the experimenter; it has however also the effect of preventing too great a condensation of the electricity in the spirals, neutralization being effected by the passage of the sparks.

Two small bars of steel move in the stands C and D, which are insulated by glass pillars. The bars are furnished at the hinder extremities with discs of brass, and at the forward extre

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