Page:Popular Science Monthly Volume 9.djvu/185

 Rh the lid: the electricities flow again together; neutrality is restored, and your lid fails to attract your balanced lath.

Once more place the lid. upon the excited surface: touch it with the finger. What occurs? You ought to know. The free electricity, which is negative, will escape through your body to the earth, leaving the chained positive behind.

Now lift the lid by the handle: what is its condition? Again I say you ought to know. It is covered with free positive electricity. If it be presented to the lath it will strongly attract it; if it be presented to the knuckle it will yield a spark.

A smooth half-crown or penny will answer for this experiment. Stick to the coin an inch of sealing-wax as an insulating handle; bring it down upon the excited India-rubber: touch it, lift it, and present it to your lath. The lath may be six or eight feet long, three inches wide, and half an inch thick; the little electrophorus-lid, formed by the half-crown, will pull it round and round. The experiment is a very impressive one.

Scrutinize your instrument still further. Let the end of a thin wire rest upon the lid of your electrophorus, under a little weight if necessary, and connect the other end of the wire with the electroscope. As you lower the lid down toward the excited plate of the electrophorus, what must occur? The power of prevision now belongs to you and you must exercise it. The repelled electricity will flow over the leaves of the electroscope, causing them to diverge. Lift the lid, they collapse. Lower and raise the lid several times, and observe the corresponding rhythmic action of the electroscope-leaves.

A little knob of sealing-wax, B, coated with tin-foil; or indeed any knob with a conducting surface, stuck into the lid of the electrophorus, will enable you to obtain a better spark. The reason of this will immediately appear.

15. Action of Points and Flames.—The course of exposition proceeds naturally from the electrophorus to the electrical machine. But before we take up the machine we must make our minds clear regarding the manner in which electricity diffuses itself over conductors, and more especially over elongated and pointed conductors.

Rub your glass tube and draw it over an insulated sphere of metal—of wood covered with tin-foil, or indeed any other insulated spherical conductor. Repeat the process several times, so as to impart a good charge to the sphere. Touch the charged sphere with your carrier, and transfer the charge to the electroscope. Note the divergence of the leaves. Discharge the electroscope, and repeat the experiment, touching, however, some other point of the sphere. The electroscope shows the same amount of divergence. Even when the greatest exactness of the most practised experimenter is brought into play, the spherical conductor is found to be equally charged at all points of its surface. You may figure the electric fluid as