in diameter. Attach, also, by means of wax to the bent arm, which ought to be about three-quarters of an inch long, two strips, I, of the Dutch metal about three inches long and from half an inch to three-quarters of an inch wide. The strips will hang down face to face, in contact with each other. In all cases you must be careful so to use your wax as not to interrupt the metallic connection of the various parts of your apparatus, which we will name an electroscope. Gold-leaf, instead of Dutch metal, is usually employed for electroscopes. I recommend the "metal" because it is less frail, and will stand rougher usage.
See that your globular flask is dry and free from dust. Bring your rubbed sealing-wax, R, or your rubbed glass, near the little plate of tin, the leaves of Dutch metal open out; withdraw the excited body, the leaves fall together. We shall inquire into the cause of this action immediately. Practise the approach and withdrawal for a little time. Now draw your rubbed sealing-wax or glass along the edge of the tin plate, T. The leaves diverge, and after the sealing-wax or glass is withdrawn they remain divergent. In the first experiment you communicated no electricity to the electroscope; in the second experiment you did. At present I will only ask you to take the opening out of the leaves as a proof that electricity has been communicated to them.
And now we are ready for Gray's experiments in a form different from his. Connect the end of a long wire with the tin plate of the electroscope; coil the other end round your glass tube. Rub the tube briskly, carrying the friction close to the coiled wire. A single stroke of your rubber, if skillfully given, will cause the leaves to diverge. The electricity has obviously passed through the wire to the electroscope.
Substitute for the wire a string of common twine, rub briskly, and you will cause the leaves to diverge; but there is a notable difference as regards the promptness of the divergence. You soon satisfy yourself that the electricity passes with greater facility through the wire than through the string. Substitute for the twine a string of silk. No matter how vigorously you rub you can now produce no divergence. The electricity cannot get through the silk at all.[1]
Mr. Cottrell, who has been recently working very hard for you and me, has devised an electroscope which we shall frequently employ in our lessons, M, Fig. 6, is a little plate of metal, or of wood covered with tin-foil, supported on a rod of glass or of sealing-wax. N is another plate of Dutch metal paper, separated about an inch from M. N I is a long straw (broken off in the figure), and A A' is
- ↑ It is hardly necessary to point out the meaning of Gray's experiment where he found that, with loops of wire or of packthread, he could not send the electricity from end to end of his suspended string. Obviously the electricity escaped in each of these cases through the conducting support to the earth.
