« PreviousContinue »
These wires carry an alternating current having a potential of 150,000 volts. The current is generated by four 17,500-kilowatt dynamos driven by 8 Pelton water wheels operating under a head of 1900 feet and developing a horse power of 100,000. Even in wet weather the under surfaces of the series of nine petticoat insulators from which each wire is hung remain sufficiently dry to prevent large leakage losses. The wires are spaced 16 feet apart
a given potential. The explanation of the increase in capacity in this case is obvious. As soon as B was brought near to A it became charged, by induction, with electricity of opposite sign to A, the electricity of like sign to A being driven off to earth through the connecting wire. The attraction between these opposite charges on A and B drew the electricity on A to the face nearest to B and removed it from the more remote parts of A, so that it became possible to put a very much larger charge on A before the tendency of the electricity on A to pass over to the electroscope became as great as it was at first, that is, before the potential of A rose to its initial value. In such a condition the electricity on A is said to be bound by the opposite electricity on B.
An arrangement of this sort consisting of two conductors separated by a nonconductor is called a condenser. If the conducting plates are very close together and one of them grounded, the capacity of the system may be thousands of times as great as that of one of the plates alone.
296. The Leyden jar. The most common form of condenser is a glass jar coated part way to the top inside and outside with tin foil (Fig. 239). The inside coating is connected by a chain to the knob, while the outside coating is connected to earth. Condensers of this sort first came into use in Leyden, Holland, in 1745. Hence they are now called Leyden jars.
FIG. 239. The Leyden jar
To charge a Leyden jar the outer coating is held in the hand while the knob is brought into contact with one terminal of an electrical machine, — for example, the negative. As fast as electrons pass to the knob they spread to the inner coat of the jar, where they repel electrons from the outer coat to the earth, thus leaving it positively charged. If the inner and outer coatings are now connected by a discharging rod,
as in Fig. 239, a powerful spark will be produced. This spark is due to the rush of electrons from the coat to the coat. Let a charged jar be placed on a glass plate so as to insulate the outer coat. Let the knob be touched with the finger; no appreciable discharge will be noticed. Let the outer coat be in turn touched with the finger; again no appreciable discharge will appear. But if the inner and outer coatings are connected with the discharger, a powerful spark will pass.
The experiment shows that it is impossible to discharge one side of the jar alone, for practically all of the charge is bound by the opposite charge on the other coat. The full discharge can therefore occur only when the inner and outer coats are connected.
Leyden jars and other forms of condensers are of great practical use. They are used, for instance, in certain systems of telephony and telegraphy, in wireless communication, and in electrostatic machines and induction coils.
297. The electrophorus. The electrophorus is a simple electrical generator which illustrates well the principle underlying the action of all electrostatic machines. All such machines generate electricity primarily by induction, not by friction. B (Fig. 240) is a hard-rubber plate which is first charged by rubbing it with fur or flannel. A is a metal plate provided with an insulating handle. When the plate A is placed upon B, touched with the finger, and then removed, it is found possible to draw a spark from it, which in dry weather may be a quarter of an inch or more in length. The process may be repeated an indefinite number of times without producing any diminution in the size of the spark which may be drawn from A.
FIG. 240. The electrophorus
If the sign of the charge on A is tested by means of an electroscope, it will be found to be positive. This proves that A has been charged by induction, not by contact with B, for it is to be remembered that the latter is charged negatively. The reason for this is that even when A rests upon B it is in reality separated from it, at all but a very few
points, by an insulating layer of air; and since B is a nonconductor, its charge cannot pass off appreciably through these few points of contact. It simply repels negative electrons to the top side of the metal plate A, and thus charges positively the lower side. The electrons pass off to earth when the plate is touched with the finger. Hence, when the finger is removed and A lifted, it possesses a strong positive charge. Every commercial electrostatic machine is simply a continuously acting electrophorus which generates electricity by induction, not by friction.
QUESTIONS AND PROBLEMS
1. If you set a charged Leyden jar on a cake of paraffin, why can you not discharge it by touching one of the coatings?
2. Will a solid sphere hold a larger charge of electricity than a hollow one of the same diameter?
3. Why cannot a Leyden jar be appreciably charged if the outer coat is insulated?
4. With a stick of sealing wax and a piece of flannel, in what two ways could you give a positive charge to an insulated body?
5. Explain, using a set of drawings, the charging of the cover of an electrophorus.
6. Represent by a drawing the electrical condition of a tower just before it is struck by lightning, assuming the cloud at this particular time to be powerfully charged with + electricity.
7. When a negatively electrified cloud passes over a house provided with a lightning rod, the rod discharges positive electricity into the cloud. Explain.
ELECTRICITY IN MOTION *
DETECTION OF ELECTRIC CURRENTS
298. Electricity in motion produces a magnetic effect. Let a powerfully charged Leyden jar be discharged through a coil which surrounds an unmagnetized knitting needle, insulated by a glass tube, in the manner shown in Fig. 241, the compass needle being at rest in the position shown. After the discharge the knitting needle will be found to be distinctly magnetized. If the sign of the charge on the jar is reversed, the direction of deflection and the poles will in general be reversed.
The experiment shows that there is a definite connection between electricity and magnetism. Just what this connection is we do not yet know with certainty, but we do know that magnetic effects are always observable near the path of a moving electrical charge, while no such effects can ever be observed near a charge at rest.
FIG. 241. Magnetic effect of an electric current produced from a static charge
To prove that a charge at rest does not produce a magnetic effect, let a charged body be brought near a compass needle. It will attract either end of the needle with equal readiness. While the needle is deflected, insert between it and the charge a sheet of zinc, aluminium, brass, or copper. This will act as an electric screen and will therefore cut off all effect of the charge. The compass needle will at once swing back to its north-and-south position.
*This chapter should be accompanied or, better, preceded by laboratory experiments on the simple cell and on the magnetic effects of a current. See, for example, Experiments 28, 29, and 30 of the authors' Manual.