294 



LESSONS IN ELECTRICITY. 



FIG. 7. 



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. 

 Stic;c by sealing-wax upon the other end of 

 the wire a little plate of tin or sheet-zinc, 

 T, about two inches in diameter. In all 

 eases you must be careful so to use your 

 wax as not to interrupt the metallic con- 

 nection of the various parts of your ap- 

 paratus, which we will name an electro- 

 scope. Gold-leaf, instead of Dutch metal, 

 is usually employed for electroscopes. 

 I recommend the " metal " because 

 it is cheaper, and will stand rougher 

 usage. 



See that your globular flask is dry and 

 free from dust. Bring your rubbed seal- 

 ing-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 im- 

 mediately. Practise the approach and 

 withdrawal for a little time. Now draw 

 \our lubbed 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 glass is withdrawn they remain divergent.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 communi- 

 cated 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 ex- 

 periments in a form different from his. 

 Connect the end of a lung 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 sino-le stroke 

 of your rubber, if skilfully given, will 

 cause the leaves to diverge. ' The elec- 

 tricity has obviously passed through the 

 wire to the electroscope. 



Substitute for the wire a string o f 

 common twine, rub briskly and youwill 

 cause the leaves to diVerge ; but 'there is 

 a notable difference us regards the 

 promptness of the divergence. '.You soon 

 satisfy yourself that the electricity , a-ses 

 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. 



This is the place to demonstrate in a 

 manner never to be forgotten the influ- 

 ence of moisture. Wet your dry silk 

 string throughout, and squeeze it a little 

 so that the water from it may not trick 1 . 3 

 over your glass tube. Coil it round the 

 tube as before, and excite the tube. The 

 leaves of the electroscope immediately 

 diverge. The water is here the con- 

 ductor. The influence of moisture was 

 first demonstrated by Du Fay (1733 to 

 1737), who succeeded in sending elec- 

 tricity through 1256 feet of moist pack- 

 thread. 



It is hardly necessary to point out the 

 meaning of Gray's experiment where he 

 found that, with loops of wire or of 

 pack-thread, he could not send the elec- 

 tricity from end to end of his suspended 

 string. Obviously the electricity escaped 

 in each of these cases through the con- 

 ducting support to the earth. 



My assistant, Mr. Cotixell, who has 

 been working very hard for you and me, 

 has devised an electroscope which we 

 shall frequently employ in our lessons. 

 M, fig. 8, is a little plate of metal, or of 

 wood covered with tin-foil, supported on 

 a rod, G, of glass or of sealing-wax. N 

 is another plate of Dutch metal paper, 

 separated about an inch from M, and at- 

 tached by sealing wax to the long straw 

 i i' (broken off in the figure) ; A A' is a 

 horizontal pivot formed by a sewing 



