Experiment 83. -Rub a glass rod briskly with silk and place in a wire sling such as was used in Experiment 12. Bring toward one end of the glass rod another glass rod which has been rubbed with silk. Do the rods attract or repel each other? Bring toward the suspended rod a piece of sealing wax or a vulcanite rod which has been rubbed with flannel or a cat"s skin. Does this repel or attract the glass rod?
Experiment 84. -Suspend a pith ball by a silk thread from the ring of a ring stand. Rub a glass rod with a piece of silk and bring it near the pith ball but do not allow the two to touch. Note the action of the ball. Touch the pith ball with the rod. Does it behave now as it did before? Rub a vulcanite rod with a piece of flannel or cat"s skin and bring it near a suspended pith ball. Does the pith ball act as it did with the glass rod? Touch the pith ball with the rod. How does it act? Bring a glass rod rubbed with silk near a pith ball which has been in contact with a vulcanite rod after it was rubbed with flannel or a cat"s skin. Does the glass rod repel or attract the ball?
Experiment 85. -Suspend a pith ball from the ring of a ring stand by a very fine piece of copper wire no larger than a thread. Wrap the wire around the pith ball in several directions. Bring a rubbed glass rod toward the pith ball. Does it act as it did when suspended by silk? Allow the ball to touch the rod. Does the ball now act as it did when suspended by silk? Try these same experiments, using the vulcanite rod.
It was known by the ancient Greeks that when certain substances, one of which was amber, were rubbed, they had the power of attracting light objects. This property was afterward called electricity, from the Greek word for amber. From the previous experiments it has been seen that when glass is rubbed with silk, and vulcanite with flannel or a cat"s skin, they seem to have two different kinds of electrical charges. The like kinds repel each other and the opposite kinds attract. These two kinds are called positive and negative respectively.
Whether there are really two kinds of electricity has not yet beenfully determined, but electricity acts exactly as it would if there were two kinds, and it has become customary to speak as if there were. In Experiment 84 it was found that pith balls suspended by a silk thread could be charged with electricity if brought in contact with a charged body. Experiment 85 showed that this was not possible when they were suspended by a copper wire. The wire conducted the electricity away. Substances like copper that conduct electricity are called conductors, and those substances like silk which will not conduct it, nonconductors.
Experiment 86. -Having started the electrical action in a static electrical machine (Fig. 75), pull the knobs as far apart as the spark will jump and notice the course taken by the spark. Does ittravel in a straight line? Hold a piece
of cardboard between the knobs so that its edge is just within the line joining them. What effect does the cardboard have upon the direction taken by the spark? Place the cardboard so that itentirely covers one of the knobs. Is the Fig. 75.
spark able to pass through the card? Attach a wire with a sharp point to each of the knobs and extend it vertically two or three inches above the knob. Start the machine. Do sparks now jump across between the knobs? Why are houses provided with lightning rods?
A FLASH OF LIGHTNING.
Showing it takes different paths of least resistance.
About the middle of the eighteenth century, Benjamin Franklin proved by his notable kite experiment that lightning was simply an electrical discharge between the clouds and the earth, or between different clouds. This discharge is similar to that which takes place on an electrical machine. The electricity in the cloudsattracts as close as possible the opposite kind of electricity on the earth"s surface and tends to hold it accumulated on high objects. If the attraction is sufficient, the electricity discharges between the cloud and the object, and we say the object was struck by lightning.
If a sharp point, such as a lightning
rod, is present on the object where the electricity tends to accumulate, it allows the electricity to pass off gradually before enough accumulates to cause damage. Lightning rods, however, must be continuous conductors and properly terminated in the ground.
80.Thunder-storms. -Often on a
hot, sultry summer afternoon large cu-
mulus clouds are seen to rise and spread
TREE COMPLETELY SHATTERED BY A
STROKE OF LIGHTNING.
out till they cover the sky. The wind soon begins to blow quite strongly toward the cloud-covered area, the clouds moving in a direction op- posite to the surface wind. As the storm clouds approach, a violent blast of wind, often called the thunder squall, blows out from the front of the storm. Soon flashes of lightning appear and thunder is heard.
As the storm comes nearer, the rain begins to descend and for a short time, usually about half an hour, it rains heavily. Then the clouds roll away and the sky becomes clear with perhaps a rainbow to heighten the beauty of the clearing landscape.
Thunder-storms are caused by hot moist air rising over certain areas and causing an updraft, which is increased by the inflow and upward movement of air from the surrounding regions. The condensation of the moisture in the rising air quickly forms clouds, and these become charged with electricity. As the electrical charge increases, discharges take place which cause lightning flashes. These discharges occur along the lines of least resistance and are often very irregular and forked. As tall objects are likely to offer good paths for the discharge, it is safest to keep away from trees and walls during a thunder-storm.
The air becomes greatly agitated by the lightning discharges and makes us aware of this by the noise of the thunder, just as the agitation of the air caused by the discharge of a gun is made apparent to us byTHUNDER-STORM CLOUDS.