CHARGE BY CASCADE.

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623

ences V1— V2, V2— V3 . . . V„—0, we obtain V1, which is accordingly equal to n times, and we have

If we compare the charges of these jars with the charge which the first jar would have received if its outer coating had been connected to earth in the ordinary way, the prime conductor being supposed to attain the same potential V1 in both cases, we have

1

for each jar in the series, whereas we should have had Q' = CV1 for the single jar. The charge of each jar in the series is therefore

As regards energy; for the single jar the energy would be Q'V1= CV12, while for any one jar in the series the energy would be

Jars thus arranged are said to be charged by cascade, the name being suggested by the successive falls of potential from jar to jar. They can either be discharged in succession by connecting the two coatings of each, or all together by connecting the inner coating of the first with the outer coating of the last. In the former case the energy of each spark is CV, as appears from the above calcula

n

tion. In the latter case the energy of the single spark is CV,2.

CHAPTER XLVIII.

EFFECTS PRODUCED BY THE DISCHARGE OF CONDENSERS.

633. Discharge of Batteries.-The effects produced by the discharge of a Leyden jar or battery differ only in degree from those of an ordinary electric spark. The shock, which is smart even with a small jar, becomes formidable with a large jar, and still more with a battery of jars.

If a shock is to be given to a number of persons at once, they must form a chain by holding hands. The person at one end of the chain

must place his hand on the outer coating of a charged jar, and the person at the other end must touch the knob. The shock will be felt by all at once, but somewhat less severely by those in the centre.

The coated pane, represented in Fig. 392, is simply a condenser, consisting of a pane of glass, coated on both sides, in its central portion, with tin-foil. Its lower coating is connected with the earth by

HEATING METALLIC THREADS.

625

a chain, and a charge is given to its upper coating by the machine. When it is charged, if a person endeavours to take up a coin laid upon its upper face, he will experience a shock as soon as his hand. comes near it, which will produce involuntary contraction of his arm, and prevent him from taking hold of the coin.

634. Heating of Metallic Threads. The discharge of electricity through a conducting system produces elevation of temperature, the amount of heat generated being the equivalent of the potential energy which runs down in the discharge, and which is jointly proportional to quantity of electricity and difference of potential. The incandescence of a fine metallic thread can be easily produced by the discharge of a battery. The thread should be made to connect the knobs ab of an apparatus called a universal discharger (Fig. 393);

these knobs being the extremities of two metallic arms supported on glass stems. One of the arms is connected with the external surface of the battery, and the other arm is then brought into connection with the internal surface by means of a discharger with glass handles. At the instant of the spark passing, the thread becomes red-hot, melts, burns, or volatilizes, leaving, in the latter cases, a coloured streak on a sheet of paper c placed behind it. When the thread is of gold, this streak is purple, and exactly resembles the marks left on walls when bell-pulls containing gilt thread are struck by lightning.

635. Electric Portrait.-The volatilization of gold is employed in producing what are called electric portraits. The outline of a por

trait of Franklin is executed in a thin card by cutting away narrow strips. Two sheets of tin-foil are gummed to opposite edges of the card, which is then laid between a gold-leaf and another card. The whole is then placed in a press (Fig. 394), the tin-foil being allowed

to protrude, and strong pressure is applied. The press is placed on the table of the universal discharger, and the two knobs of the

latter are connected with the two sheets of tin-foil. The discharge is then passed, the gold is volatilized, and the vapour, passing through the slits to the white card at the back, leaves purple traces which reproduce the design. 636. Velocity of Electricity. Soon after the invention of the Leyden jar, various attempts were made to determine the velocity with which the discharge travels through a conductor connecting the two coatings. Watson, about 1748, took two iron wires, each more than a mile long, which he arranged on insulating supports in such a way that all four ends were near together. He held one end of each wire in his hands, while the other ends were connected with the two coatings of a charged jar. Although the electricity had more than a mile to travel along each wire before it could reach his hands, he could never detect any interval of time between the passage of the spark

VELOCITY OF ELECTRICITY.

627

from the knob of the jar and the shock which he felt. The velocity was in fact far too great to be thus measured.

Wheatstone, about 1836, investigated the subject with the aid of the revolving mirror of which we have spoken above (§ 591). He connected the two coatings of a Leyden jar by means of a conductor which had breaks in three places, thus giving rise to three sparks. When the sparks were taken in front of the revolving mirror, the positions of the images indicated a retardation of the middle spark, as compared with the other two, which were taken near the two coatings of the jar, and were strictly simultaneous. The middle break was separated from each of the other two by a quarter of a mile of copper wire. He calculated that the retardation of the middle spark was of a second, which was therefore the time occupied in travelling through a quarter of mile of copper wire. This is at the rate of 288,000 miles per second, a greater velocity than that of light, which is only about 186,000 miles per second.

1 1,152,000

Since the introduction of electric telegraphs, several observations have been taken on the time required for the transmission of a signal. For instance, trials in Queenstown harbour, in July, 1856, when the two portions of the first Atlantic cable, on board the Agamemnon and Niagara, were for the first time joined into one conductor, 2500 miles long, gave about 12 seconds as the time of transmission of a signal from induction coils, corresponding to a velocity of only 1400 miles per second. In 1858, before again proceeding to sea, a quicker and more sensitive receiving instrument-Thomson's mirror galvanometer-gave a sensible indication of rising current at one end of 3000 miles of cable about a second after the application of a Daniell's battery at the other.

It seems to be fully established by experiment that electricity has no definite velocity, and that its apparent velocity depends upon various circumstances, being greater through a short than through a long line, greater (in a long line) with the greater intensity and suddenness of the source, greater with a copper than with an iron wire, and much greater in a wire suspended in air on poles than in one surrounded by gutta-percha and iron sheathing, and buried under ground or under water. In a long submarine line, a short sharp signal sent in at one end, comes out at the other as a signal gradually increasing from nothing to a maximum, and then gradually dying

away.