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286. Charging the electroscope by induction. Let an ebonite rod which has been rubbed with catskin be brought near the knob of the electroscope (Fig. 223). The leaves at once diverge. (Make a diagram of the electroscope with the negatively charged ebonite rod near the knob. By use of + and signs explain the electrical condition of both the knob and the leaves.) Let the knob be touched with the finger while the rod is held in place. The leaves will fall together. (Explain by a diagram as before.) Let the finger be removed and then the rod. The leaves will fly apart again. (By a diagram explain the final electrical condition of both the knob and the leaves.)
The electroscope has been charged by induction, and since the charge on the ebonite rod was negative, the charge on the electroscope must be positive. If this conclusion is tested by bringing the charged ebonite rod near the electroscope, the leaves will fall together as the rod approaches the knob. How does this prove that the charge on the electroscope is positive? If the empty neutral hand approaches the knob, the leaves diverge less. Explain.
287. Plus and minus electricities always appear simultaneously and in equal amounts. Let an ebonite rod be completely discharged by passing it quickly through a Bunsen flame. Let a flannel cap having a silk thread attached be slipped over the rod, as in Fig. 227, and twisted rapidly around a number of times. When rod and cap together are held near a charged electroscope, no effect will be observed; but if the cap is pulled off, it will be found to be positively charged, while the rod will be found to have a negative charge.
FIG. 227. Plus and
minus electricities always developed
in equal amounts
Since the two together produce no effect, the experiment shows that the plus and minus charges were equal in amount. This experiment confirms the view already brought forward in connection with induction, that electrification always consists in a separation of plus and minus charges which already exist in equal amounts within the bodies in which the electrification is developed.
QUESTIONS AND PROBLEMS
1. If pith balls, or any light figures, are placed between two plates (Fig. 228), one of which is connected to earth and the other to one knob of an electrical machine in operation, the figures will bound back and forth between the two plates as long as the machine is operated. Explain. 2. Given a gold-leaf electroscope, a glass rod, and a
piece of silk, how, in general, would you proceed to test the sign of the electrification of an unknown charge?
3. Charge a gold-leaf electroscope by induction from a glass rod. Warm a piece of paper and stroke it on the clothing. Hold it over the charged electroscope. If the divergence of the gold leaves is increased, is the charge on the paper + or -? If the divergence is decreased, what is the sign of the charge on the paper?
4. If you are given a positively charged insulated sphere, how could you charge two other spheres, one positively and the other negatively, without diminishing the charge on the first sphere? 5. If you bring a positively charged glass rod near the knob of an electroscope and then touch the knob, why do you not remove the negative electricity which is on the knob?
6. In charging an electroscope by induction, why must the finger be removed before the removal of the charged body?
7. If you hold a brass rod in the hand and rub it with silk, the rod will show no sign of electrification; but if you hold the brass rod with a piece of sheet rubber and then rub it with silk, you will find it electrified. Explain.
8. State as many differences as you can between the phenomena of magnetism and those of electricity.
9. If an electrified rod is brought near to a pith ball suspended by a silk thread, the ball is first attracted to the rod and then repelled from it. Explain this.
DISTRIBUTION OF ELECTRIC CHARGE UPON CONDUCTORS
288. Electric charges reside only upon the outside surface of conductors. Let a deep tin cup (Fig. 229) be placed upon an insulating stand and charged as strongly as possible either from an ebonite rod or from an electrical machine. If now a smooth metal ball suspended by a silk thread is touched to the outside of the charged cup and then brought near the knob of a charged electroscope, it will show a strong charge; but if it is touched to the inside of the cup, it will show no charge at all.
These experiments show that an electric charge resides entirely on the outside surface of a conductor. This is a result which might have been inferred from the fact that all the little electrical charges of which the total charge is made up repel each other and therefore move through the conductor until they are, on the average, as far apart as possible.
FIG. 229. Proof that charge resides on surface
289. Density of charge greatest where curvature of surface is greatest, Since all of the parts of an electric charge tend, because of their mutual repulsions, to get as far apart as possible, we should infer that if a charge of either sign is placed upon an oblong conductor like that of Fig. 230, (1), it will distribute itself so that the electrification at the ends will be stronger than that at the middle.
To test this inference let a proof plane-a flat metal disk (for example, a cent) provided with an insulating handle — be touched to one end of such a charged body, the charge conveyed to a gold-leaf electroscope, and the amount of separation of the leaves noted. Then let the experiment be repeated when the proof plane touches the middle of the body. The separation of the leaves in the latter case will be found to be very much less than in the former. If we should test the distribution on a pear-shaped body (Fig. 230, (2)) in the same way, we should find the density of electrification considerably greater on the small end than on the large one. By density of electrification is meant the quantity of electricity on unit area of the surface.
FIG. 230. Distribution of charge over oblong bodies
290. Discharging effect of points. The above experiments. indicate that if one end of a pear-shaped body is made more and more pointed, then, when the body is charged, the electric
density on this end will become greater and greater. The following experiment will show what happens when the conductor is provided with a sharp point.
Let a very sharp needle be attached to any smooth insulated metal body provided with paper or pith-ball indicators, as in Fig. 224, p. 229. If the body is now charged either with a rubbed rod or with an electric machine, as soon as the supply of electricity is stopped the paper indicators will immediately fall, showing that the body is losing its charge. To show that this is certainly due to the effect of the point, remove the needle and repeat. The indicators will fall very slowly if at all.
The experiment shows that the electrical density upon the point is so great that the charge escapes from it into the air. This is because the intense charge on the point causes many of the adjacent molecules of the air to lose an electron. This leaves these molecules positively charged. The free electrons attach themselves to neutral molecules, thus charging them negatively. One set of these electrically charged molecules (called ions) is attracted to the point and the other repelled from it. The former set move to the conductor, give up their charges to it, and thus neutralize the charge upon it.
The effect of points may be shown equally well by charging the goldleaf electroscope and holding a needle in the hand within a few inches of the knob. The leaves will fall together rapidly. In this case the needle point becomes electrified by induction and discharges to the knob electricity of the opposite kind to that on the knob, thus neutralizing its charge. An entertaining variation of the last experiment is to attach a tassel of tissue paper to an insulated conductor and electrify it strongly. The paper streamers under their mutual repulsions will stand out in all directions, but as soon as a needle point is held in the hand near them, they will fall together (Fig. 231), being discharged as described above. 291. The electric whirl. Let an electric whirl (Fig. 232) be balanced upon a pin point and attached to one knob of an electric machine. As soon as the machine is started, the whirl will rotate rapidly in the direction of the arrows.
FIG. 231. Discharging effect of points
The explanation is as follows: The air close to each point is ionized, as explained in § 290. The ions of sign unlike that of the charge on the point are drawn to the point and discharged. The other set
of ions is repelled. But since this repulsion is mutual, the point is pushed back with the same force with which these ions are pushed forward; hence the rotation. The repelled ions
FIG. 232. The
FIG. 233. The elec
in their turn drag the air with them in their forward motions and thus produce the "electric wind," which may be detected easily by the hand or by a candle flame (Fig. 233).
292. Lightning and lightning rods. It was in 1752 that Franklin (see opposite p. 230), during a thunderstorm, sent up his historic kite (see opposite p. 231). This kite was provided with a pointed wire at the top. As soon as the hempen kite-string had become wet he succeeded in drawing ordinary electric sparks from a key attached to the lower end. This experiment demonstrated for the first time that thunderclouds carry ordinary electrical charges which may be drawn from them by points, just as the charge was drawn from the tassel in the experiment of § 290. It also showed that lightning is nothing but a huge electric spark. Franklin applied this discovery in the invention of the lightning rod. The way in which the rod discharges the cloud and protects the building is as follows: As the charged cloud approaches the building it induces an opposite charge in the rod. This induced charge escapes rapidly and quietly from the sharp point in the manner explained above and thus neutralizes the charge of the cloud.
To illustrate, let a metal plate C (Fig. 234) be supported above a metal ball E, and let C and E be attached to the two knobs of an electrical machine. When the machine is started, sparks will pass from C to E.