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treatise already mentioned, in § 64, adds, in reference to this fact, the following observations :

When, in forming the arc, a positive metallic point is opposite to a negative plate, the point becomes ignited throughout, while on inverting the poles the negative point is heated at its extremity only.

If two points of the same metal are opposed to each other the positive one becomes more intensely ignited, and over a greater length. If they are of different metals, of course that one becomes most intensely ignited which is made of the worst conducting metal.

To this category belongs also an observation of Walker, made with a Daniell's battery of 160 cups.-(Trans. of the Lond. Electr. Soc., pp. 65 and 71; Pog: Ann., LV, 62.) He laid the pole wires crosswise, but so that after the contact they were again moved to a little distance from each other, and a short arc of light passed between them. Under these circumstances the positive end of the wire, from the point of crossing, became so intensely hot that it softened and bent, while the negative end remained comparatively cold.

Experiments on the heating effects of the voltaic arc have been made on the greatest scale by Despretz. He collected, in Paris, 500 zinc-carbon cups, and arranged a battery of 124 elements, each consisting of four Bunsen's cups. When a piece of sugar carbon, in a glass globe exhausted to 5 millimetres, was brought between the poles it became intensely ignited and the globe was covered with a dry, crystalline black powder. Carbon from gas retorts produced the same effects. This shows a sublimation of the carbon.

Despretz thinks too that he observed traces of fusion of the carbon. At any rate his experiments show that carbon evaporates more readily than it melts. He believes that it could be melted in metallic vessels in an atınosphere of compressed nitrogen. Similar in behavior to carbon are lime, magnesia, oxide of zinc, &c. Alumina, rutil, anatase, nigrine, oxide of iron, &c., form at first small globules, but afterwards evaporate.

Previous to these experiments with 496 cups Despretz had used a battery of 165 elements, and combined the heat of its arc with that of the oxy-hydrogen blow-pipe and of the sun concentrated through a sec. tional lens 90 centimetres in diameter. The effect of the galvanic battery was increased by the addition of the other sources of heat.(Comptes Rendus, July, 1849, No. 3 ; Dingler's Polytechnic Journal, CXIV, 342.)

$ 67. Influence of magnetism upon the voltaic arc.-That magnetic forces have an influence upon the position and form of the arc has already been observed by Davy, and it is known that this arc is affected by a magnet in the same manner as a movable conductor when a galvanic current is passing through it; the terrestrial magnetism, therefore, must also act upon it. By the motion of the heated air the arc of light is always carried upwards, so as to form a curve, convex above. If we conceive a perpendicular plane to be passed through the carbon points lying horizontally, the action of terrestrial magnetism will be such that the highest point of the arc will never be in this plane, but on one side or the other.

Casselmann, in his treatise already mentioned, in $ 56, gives experiments on this subject. If, with opposite horizontal carbon points, the current was passing

From The deviation of the apex was towards
N. to S.

E.
W. to E.

N.
S. to N.

W.
E. to W.

S. This can be easily deduced. In fig. 61 a and b represent the two horizontal carbon points between which the

Fig. 61. arc is produced. If now we imagine a perpendicular plane passed through a and b, and a straight line to pass perpendicularly through the plane between these points, as indicated by the arrow, then a steel needle placed in this line would be magnetized by the current of the arc, and its N. end would be at the point of the arrow when the positive current is passed from a through the arc towards b. But by the influence of the terrestrial magnetism the N. end of the needle would dip, and in like manner also the arc will be inclined from the vertical plane towards the direction of the N. end of the needle.

If a is to the west, and b east, the inclination will be toward the north when the current is passing from a to b; but with a direction of the current from east to west, the north end of the supposed magnetic needle would be on the south side of the arc, and the latter, therefore, would incline toward the south.

By means of this supposed magnetic needle we can, under all circumstances, determine in what manner the arc will be affected by terrestrial magnetism or either pole of a magnet, or what must be its position when placed between the two poles of a horse-shoe magnet.

If, instead of one of the carbon poles, a magnetic bar is used, so that the arc is formed between carbon and steel, the arc rotates around the magnetic pole according to the same laws which apply when a movable current rotates around a fixed magnet. The first notice in reference to this rotation of the arc is given by Walker, in the “ Transactions of the London Electrical Society" from 1837 to 1840.-(Pog. Ann., LIV, 514.) De la Rive also has made experiments on the influence of magnetism upon the voltaic arc, but in a different way. Their description is found in the memoir mentioned already in § 64.

I shall quote here from De la Rive's memoir literally, in order to give a characteristic example of his want of precision in writing, by which his papers are frequently rendered obscure, as before mentioned:

“ If two points of soft iron, acting as electrodes, be both placed within a helix formed of thick copper wire of several coils, the voltaic arc developed between the two points of iron ceases the moment a strong current is passed through the wire of the helices, and reappears if this current be arrested before the points have become cold. The arc cannot be formed between the two iron points when they are mag. netized, whether by the action of the helices or by that of a powerful

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magnet, unless they be brought much nearer to one another, and the appearance of the phenomenon is then entirely different. The transported particles appear to disengage themselves with difficulty from the positive electrode, sparks fly with noise in all directions, while in the former case it was a vivid light without sparks and without noise, accompanied by the transfer of a liquid mass, and this appeared to be effected with the greatest ease.

It is of little moment with respect to the result of the experiment whether the two rods of magnetized iron present to that part of their extremities between which the luminous arc springs the same magnetic poles or different poles.

"The positive electrode of iron, when it is strongly magnetized, produces, the moment that the voltaic arc is formed between it and a negative electrode of whatever nature, a very intense noise, analogous to the sharp hissing sound of steam issuing from a locomotive engine. This noise ceases simultaneously with the magnetization.

“For the purpose of better analyzing these different phenomena, I placed an electro-magnet of large dimensions and great power in such a manner as to enable me to place on each of its poles, or between them, different metals destined to form one of the electrodes of the pile, while one point of the same metal, or another substance, acted as the other electrode. I have alike employed as electrodes, placing them in the same circumstances, two points of the same meta), or of different metals. The following are the results which I have obtained : A plate of platinum was placed on one of the poles of the electro-magnet, and a point of the same metal was placed vertically above it; the voltaic arc was produced between the plate and the point, the plate being positive and the point negative. As soon as the electro-magnet was charged a sharp hissing was heard. It became necessary to bring the point nearer to the plate to enable the arc to continue, and the bluish circular spot which the platinum plate presented became larger than when the experiment was made beyond the influence of the electro-magnet.

† “ The plate was made negative, and the point positive. The effect was then totally different. The luminous arc no longer maintained its vertical direction when the electro-magnet was charged, but took an oblique direction, as if it had been projected outwards towards the margin of the plate. If It was broken incessantly, each time accompanied by a sharp and sudden noise, similar to the discharge of a Leyden jar. The direction in which the luminous arc is projected depends upon the direction of the current producing it, as likewise on the position of the plate on one or other of the two poles, or between the poles of the electro-magnet. A plate and a point of silver, a plate and a point of copper, and generally a plate and a point of any other metal, provided it be not metal too easily fused, present the same phenomena.

“Copper, and still more silver, present a remarkable peculiarity. Plates of these two metals retain on their surfaces the impression of the action that took place in the experiments just described. Thus, when the plate is positive, that portion of its surface lying beneath the negative point presents a spot in the form of a helix, as if the melted metal in this locality had undergone a gyratory motion around a centre, at the

same time that it was uplifted in the shape of a cone towards the point.”

The first part of this is clear; not so the last two paragraphs. The passage between † and tt appears to indicate that the oblique direction of the arc of light only occurs when the plate is negative and the point positive; but somewhat further on we read that the direction in which the luminous arc is projected depends upon that of the exciting current. It should, therefore, take place when the plate is positive and the point negative. Besides, an obscure allusion to the rotation of the arc is found in this passage, but so obscure that one not previously acquainted with the phenomenon could form no idea of it from this representation. That the Genevan physicist, in penning this passage, actually had this rotation in view is evident from the conclusion of the last paragraph. Similar faults frequently occur in De la Rive's treatises ; his description rarely gives a clear and intelligible representation of the phenomenon. It is much to be regretted that in this way the results of many a beautiful and difficult experimental research are only imperfectly presented to those engaged in physical studies.

$ 68. Use of the galvanic light for illumination. It was to be expected that the great intensity of the galvanic carbon light would soon lead to the idea of employing it for illumination after its production was so much facilitated by the invention of tlie constant batteries.

Deleuil several times made public experiments with this kind of illumination. At first he illuminated the pavilion of a mansion at the Pontneuf, in Paris, with 98 zinc-carbon elements. Acherau made similar experiments in the Place de la Concord.—(Dingler's Polytech. Journal, vol. 91, p. 324.)

Though the intensity of the galvanic carbon light is enormous, and although a battery of 48 Bunsen's elements produces as much light as 63 common gas burners, yet the use of the galvanic light for public illumination appears unfit for practical application for the following reasons :

An immense quantity of light is here emanating from one single point, and therefore very strong contrasts between light and shade will be produced; the darkness in the shade will be the more unpleasant just on account of its contrast to the dazzling light. At any rate, the illumination obtained from 63 gas burners, perfectly distributed, will be more uniform and agreeable than an equivalent light concentrated in one point.

Another objection to the application of the galvanic carbon light, is the difficulty of keeping its intensity uniform for a long time.

In consequence of the formation of sulphate of zinc the conducting power of the fluid decreases so rapidly that the force of the current, even in half an hour, becomes considerably weaker than it was at the beginning. But, apart from this, the maintenance of the battery is extremely expensive, because much more zinc is consumed than the current itself requires, and the nitric acid acts destructively upon the metallic rings around the carbon cylinders. It is true the disadvantages of this action of the nitric acid could be avoided by the use of Daniell's elements, but then the battery must be considerably enlarged to obtain the same effect.

T

In an economical point of view, therefore, the galvanic illumination of streets, halls, theatres, &c., does not appear advantageous. But there is yet another difficulty; the management of the battery and of the whole apparatus is too complicated to be confided to such persons as generally have charge of the illumination; the carbon points are continually changing, and their position, therefore, must be continually regulated in order to keep the light uniform and prevent its extinction. It is difficult to accomplish this regulation by mechanical means, though different contrivances have been proposed for the purpose. Le Molt, for instance, obtained a patent in England, in 1848, for an apparatus for galvanic illumination, in which carbon Fig. 62.

disks, with the form represented in fig. 62, take the place of the points. Two of these disks are placed with their sharp edges opposite each other; their axes rotate uniformly by means of clock work, and their distances are regulated by a metallic spring.

It is therefore scarcely to be expected that the applica

tion of galvanism to public illumination will have any practical success. But Donné and Foucault have obtained very favorable results from their experiments, in which the galvanic carbon light was substituted for the incandescent lime in the so called gas microscope.

A tolerably complete description of the photo-electric microscope of Donné and Foucault may be found in the 4th edition of Pouillet, Elements de Physique Experimentale, &c., vol. II, pp. 746. We can here only indicate the most essential parts of the apparatus. The luminous arc is produced between sticks of carbon cut from the hard

carbon of gas retorts; they are made in the shape represented in fig. 63, the negative electrode being pointed and the positive blunt. These carbon pieces are so held that their position can

easily be regulated. A general idea of the arrangement of the illuminating apparatus of this microscope may be gathered from the diagram fig. 64. a is a

concave mirror of an aperture of about i decimetre, and a radius of 1.6 decimetre. The carbon light is at b, a little nearer to the mirror than c, the centre of its curvature, and somewhat higher, so that the rays emanating from b are collected at f, where the minute object to be magnified is intensely illuminated. The system of lens through which the magnified

image of the object is thrown upon a screen 4 to 5 metres distant is precisely the same as in the solar microscope.

To diminish the great heat at f, a vessel is placed between the mirror and b, the sides of which are made of polished plate glass; it it is filled with a solution of alum by which a great part of the calo

Fig. 63.

Fig. 64.

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