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Fig. 74.

Fig. 75.

“Take next a drop of strong solution of muriate of lime; being electrified, a part will probably be dissipated, but a-considerable portion, if the electricity be not too powerful, will remain, forming a conical drop, (fig. 74,) accompanied by a strong wind. If glow be produced the drop will be smooth on the surface. If a short low brush is formed a minute tremulous motion of the liquid will be visible.”

“ With a drop of water the effects were of the same kind, and were best obtained when a portion of gum water or syrup hung from a ball, (fig. 75.) When the machine was worked slowly a fine, large, quite conical drop, with concave lateral outline and a small rounded end, was produced, on which the glow appeared, whilst a steady wind issued from the point of the cone of sufficient force to depress the surface of uninsulated water held opposite to the termination. When the machine was worked more rapidly some of the water was driven off, the smaller pointed portion left was roughish on the surface, and the sound of successive brush discharges was heard. With still more electricity, more water was dispersed; that which remained was alternately elongated and contracted," and "a stronger brush discharge was heard. When water from beneath was brought towards the drop, it did not indicate the same regular, strong, contracted current of air as before; and when the distance was such that sparks passed the water beneath was attracted rather than driven away, and the current of air ceased."-(1584.)

"That the drop, when of water, or a better conductor than water, is formed into a cone principally by the current of air, is shown, amongst other ways, thus: A sharp point being held opposite the conical drop, the latter soon lost its pointed form, was retracted and became round; the current of air from it ceased, and was replaced by one from the point beneath, which, if the latter was held near enough to the drop actually blew it aside and rendered it concave in form.” With still worse conductors, as oil, or oil of turpentine, the fluid was "spun out into threads and carried off, not only because the air rushing over its surface helped to sweep it away, but also because its insulating particles assumed the same changed state as the particles of air, and, not being able to discharge to them in a much greater degree than the air particles themselves could do, were carried off by the same causes which urged these in their course. A similar effect with melted sealingwaxona metal point forms an old and well known experiment.”—(1588.)

“A drop of gum water in the exhausted receiver of the air pump was not sensibly affected in its form when electrified,” which was partly owing to the diminished current of air, and partly, perhaps, that the tension of the electricity on the ball is not so great in rarefied as in dense air.

“That I many not be misunderstood,” says Faraday, “I must observe here that I do not consider the cones produced as the result only of the current of air or other insulating dielectrics over their surface. When the drop is of badly conducting matter a part of the effect is due to the electrified state of the particles,” &c.—(1594.)

“When the phenomena of currents are observed in dense insulating dielectrics they present us with extraordinary degrees of mechanical force. Thus, if a pint of well rectified and filtered oil of turpentine be put into a glass vessel and two wires be dipped into it in different places, one leading to the electrical machine and the other to the discharging train, on working the machine, the fluid will be thrown into violent motion, whilst, at the same time, it will rise 2, 3, or 4 inches up the machine wire, and dart off in jets from it into the air.”—(1595.)

A drop of mercury being suspended from an amalgamated brass ball preserved its form almost unchanged in air, but when immersed in the oil of turpentine it became very pointed and even particles of the metal could be spun out and carried off. The form of the liquid metal was just like that of syrup in air.”—(1597.)

“If the mercury at the bottom of the fluid be connected with the electrical machine, whilst a rod is held in the hand terminating, in a ball three quarters of an inch in diameter, and the ball be dipped into the electrified fluid, very striking appearances ensue. When the ball is raised again so as to be at a level nearly out of the fluid, large portions of the latter will seem to cling to it, (fig. 76.) If it be raised

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higher a column of the oil of turpentine will still connect it with that in the basin below, (fig. 77.) If the machine be excited into inore powerful action this will become more bulky, and may then also be raised higher, assuming the form, (fig. 78.)

“A very remarkable effect is produced on these phenomena, connected with positive and negative charge and discharge, namely, that a ball charged positively raises a much higher and larger column of the oil of turpentine than when charged negatively.”—(Faraday Researches, series XIII, 1600.)

$ 89. Laws of the brightness of the electrical spark.—Masson published in the 14th volume of the Anneles de Chimie et de Physique, page 129, (1845, 3d part,) his researches upon the brightness of the electrical spark, under the title : “ Etudes de Photometrie Electrique."

The ordinary photometre can be used only for permanent and not for momentary sources of light; for measuring the brightness of the electrical spark, which gives only a momentary illumination, Masson was

Fig. 79.

Fig. 80.

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obliged to contrive a new photometric principle. In fact he solved the problem in a very ingenious manner.

If a disk be divided into sectors equally large and alternately black and white, as in fig. 79, and be put into rapid rotation, the different sectors cannot be distinguished when the disk is illuminated by a constant source of light; but if it be illuminated by an electrical spark for an instant the sectors of the rotating disk will become visible again, and as much more so as the electrical spark is brighter. But if the illumination by the electrical spark be gradually weakened, while that from the constant source of light remain the same, a point will be attained where the sectors just cease to be distinguishable, and in this case the power of the illumination by the electrical spark is a determinate fraction of the illumination by the constant source, its magnitude depending upon the peculiarity of the observer's eye.

We will now consider in what manner this limit of the ability to distinguish may be ascertained.

A part of a sector on a white disk, (fig. 80,) being blackened, and the disk turned rapidly about its centre, the black piece will form a ring somewhat darker than the white ground of the disk. The ring will appear as much fainter as the black spot is narrower, and if the experiment be made with a series of such disks, each successive one having a narrower black end-portion of a sector mn, we will at last find one in which the dark ring ceases to be distinguishable.

Let us suppose this to be the case when the breadth of the sector is ido of the entire circumference; it is evident that the brightness of the ring is less than the brightness of the disk by Tho; in this case the eye cannot distinguish a difference of do in illumination.

Masson made his experiments with disks upon which the breadth of the sectors were zo, do, too go, do, ito, hoho, of the whole circumference, and by means of them he found that for weak eyes a difference of illumination of t'y to so was the limit of perceptibility. For ordinary eyes this limit was to ido; for very good eyes ī šo to 10

On varying the intensity of the illumination Masson found that the sensibility for the same individual did not change if the illumination was sufficient for reading ordinary print.

The rotating plate being illuminated with colored light, Masson found that the limit of perceptibility of difference of illumination is independent of the color.

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92

Fig. 81.

We now pass to the particular object of Masson's investigation. The arrangement of his experiments was essentially as follows: A

rotating disk, a b, fig. 81, (the rotation being produced by clock-work,) divided into white and black sectors, as in fig. 79, was illuminated in the direction of A C by the constant light of a lamp L, which was movable in the line of this direction. This lamp was placed in a black case so that it could throw its light on the rotating disk only through a tube. In the direction of the line B C a movable spark micrometre F was placed. One of the knobs of this micrometer was in conducting connexion with the upper coating of a horizontal glass plate, the other knob with the lower coating;

the spark always passed between the two knobs as soon as the charge of the plate had reached a certain limit, which depended upon the distance of the knobs from each other.

Masson first satisfied himself that, for the instantaneous light of the electrical spark, the intensity of the illumination was also, as in other cases, in the inverse ratio of the square of the distance.

The lamp L being at a given distance from the disk a b, the spark micrometer was gradually removed from the disk, until at the passage of the spark the sectors of the rotating disk were no longer distinguishable, and the distance of the spark from the disk was determined

. The lamp was then moved, and the same experiment repeated, the distance between the knobs of the spark micrometer remaining unchanged. The following table gives the results of such an experimental series ; Z denotes the distance of the lamp, Y the corresponding distance of the spark micrometer from the middle of the disk a b:

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Since Z and Y increase in an equal (or very nearly equal) ratio, it is evident that, with increasing distances, the illumination for both sources of light decreases according to the same law; hence the illomination by an electrical spark is likewise inversely proportional to the square of the distance,

The same result was given by several other series of experiments, which Masson has arranged in tables. It will be sufficient to present here only one of the many series, serving to establish each of the laws determined.

The values of Y, as given in the tables, are always the mean of two experiments. After the distance Y of the spark micrometer from the rotating disk at which the sectors could be no longer distinguished had been once determined the micrometer was brought considerably nearer the disk again, and then removed the second time, until the sector disappeared. The two values of Y, thus determined, differed in the various series at most by one centimetre, a proof of the exactness attainable by this method of observation.

§ 90. Variation of the brightness of the spark at different striking distances.– On this point Masson made numerous experiments. The following table contains the results of one of them :

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Here X denotes the striking distance, Y the corresponding distance of the spark from the rotating disk, at which the sectors cannot be distinguished, under the condition that the constant illumination of the rotating disk from AC remained unchanged during the whole series of experiments.

It is evident, from the above table, that the striking distance and the corresponding distance of the spark from the rotating disk must vary in a constant ratio, the illumination of the disk remaining the same. Or, for double and treble striking distances, he spark must be removed two and three times as far from the disk, if its illumination by the spark is to remain the same.

By doubling the distance, the intensity of the illumination becomes four times feebler, but it remains unchanged if the striking distance is doubled, consequently the brilliancy of the spark mus tbe four times greater at double the striking distance. For the distance n the illumination by the electrical spark is no times feebler, but it remains unchanged if the striking distance is made n times greater; hence, for a striking distance n the brilliancy of the spark must be n2 times greater, or in other words, the brightness of the electrical spark increases as the square of the striking distance.

$ 91. Influence of the size and form of the surface of the condenser. The form of the condenser (that is, the glass plate with metallic coating on both sides, the discharge of which passes through the spark

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