LESSONS IN ELECTRICITY. 615 



ia diameter. Attach, also, by means of wax to the bent arm, which 

 ought to be about three-quarters of an inch long, two strips, 7", 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 

 roucrher ixsasfe. 



See that your globular flask is di-y and free from dust. Bring 

 your rubbed sealing-wax, 7?, or your rubbed glass, oiear 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 wax or glass is withdrawn they remain divergent.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 experi- 

 ment 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 com- 

 municated 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 di- 

 verge. The 'electricity has obviously passed through the w4re 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 differ- 

 ence as regards the promptness of the divergence. You soon satis- 

 fy 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.' 



Mr. Cottrell, who has been recently working very hard for you 

 and me, has devised an electroscope which we shall frequently era- 

 ploy in our lessons, jlf. 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. 

 NT is another plate of Dutch metal paper, separated about an inch 

 from 31. iVZis 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. 



