LESSORS IN ELECTRICITY. 



Fw. 24. 



to know. It is covered with free posi- 

 tive 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 iniooth half-crown, or a penny, will 

 answer for this experiment. Stick to tho 

 coin an inch of sealing-wax as an insulat- 

 ing handle : bring it down upon the ex- 

 cited 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 exer- 

 cise it. The repelled electricity will flow 

 over the leaves of the electroscope, caus- 

 ing them to diverge. Lift the lid, they 

 oollapse. 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 to the lid of 

 the electrophorus, will enable you to ob- 

 tain a better spark. The reason of this 

 will immediately appear. 



More than half the ralne of your pres- 

 ent labor consists in arranging each ex- 

 periment in thought before it is realized 

 in fact ; and more than half the delight 

 of your work will consist in observing 

 the verification of what you have foreseen 

 and predicted. 



16. 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 elec- 

 tricity diffuses itself over conductors, and 

 more especially over elongated and point- 

 ed 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. Dis- 

 charge the electroscope, and repeat tho 

 experiment, touching, IIOWCVPV, some 

 other point of the sphere. The electro- 

 scope shows sensibly the same amount of 

 divergence. Even when the greatest ex- 

 actness of the most practised experi- 

 menter 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 a little ocean 

 encompassing the sphere, and of the same 

 depth everywhere. 



But supposing the conductor, instead 

 of being a sphere, to be a cube, an elon- 

 gated cylinder, a cone, or a disk. The 

 depth, or as it is sometimes called the 

 density, of the electricity, will not be 

 everywhere the same. The corners of 

 the cube will impart a stronger charge to 

 your carrier than the sides. The end of 

 the cylinder will impart a stronger charge 

 than its middle. The edge of the disk 

 will impart a stronger chargo than its flat 

 surface. The apex or point of the cone 

 will impart a stronger chargo than its 

 curved surface or i:s base. 



You can satisfy yourself of the truth of all 

 this in a rough, but certain way, by charg- 

 ing, after the sphere, a turnip cut into the 



