13S 



ELECTRICITY. 



Lesson III. 



furnished with a -winch handle (H), by 

 which it may be made to revolve. A 

 cushion or rubber (R), which is covered 

 with an amalgam,* is attached to a glass 

 pillar fitted with a sliding base (6), so as 

 to allow it to press with a greater or less 

 force against the cylinder. A flap of oiled 

 silk (s) is attached to the rubber and 

 thrown loosely over the cylinder, and just 

 where it terminates is a cross-piece of brass 

 furnished with points, corresponding to 

 one on the opposite side (A), this is con- 

 nected with an insulated cylindrical con- 

 ductor (N and p), and as the machine is 

 placed between two of these conductors, 

 one collects + , and the other electricity. 



37. T. What is the use of the silk flap? 



P. To keep up the friction on the sur- 

 face of the glass, so that the electricity 

 excited by the rubber (R) may not be lost 

 on its passage to the conductor (P). 



38. T. Can we produce positive and 

 negative electricity at will ? 



P. Yes, if we wish to have a charge of 

 vitreous or positive electricity, we put the 

 conductor (N) in connection with the earth, 

 so that electricity is continually supplied 

 to the glass cylinder. If we wish for a 

 supply of resinous or negative electricity 

 we reverse the proceeding, and remove 

 the + conductor (P), or connect it with the 

 earth, so that the electricity thrown upon 

 the glass is relieved, and a constant supply 

 of the electricity we require afforded to 

 the conductor (N). 



39. T. Is it necessary to have the ends 

 of the cylindrical machines permanently 

 closed ? 



P. No; it is better to have the ends 

 sufficiently large to admit the hand, for 

 the purpose of wiping out and drying the 

 inside, because if they are closed, con- 

 densation takes place on the inside and 

 prevents the proper action of the machine, 

 and therefore the ends should be fitted 

 with wooden caps which n't the openings 

 exactly, and may be removed at any time. 



40. T. Describe the plate-glass elec- 

 trical machine. 



P. It consists of a circular plate o 



* This is made by melting two ounces of zin 

 m an iron ladle and then adding one ounce of 

 tin, after which four ounces of heated mercury 

 should be gradually poured into tlie ladle and 

 the whole well stirred, and when nearly solid the 

 amalgam should be poured into some vessel 

 and agitated BO r,8 to reduce it to powder. 



Fig. 13.* Fiy. . 



jlass, the diameter of which of course 

 varies, some plates being only 9 inches, 

 and others 80 inches in diameter. An axis 

 passes through an opening in its centiv, 

 to one end of which is a winch-handle, and 

 the whole is supported by the frame- work 

 to which the rubbers are attached. This 

 machine is furnished with a conductor, 

 which, like that of the cylindrical one, is 

 insulated, and has ends with points plac'-.i 

 opposite to the termination of the silk 

 flaps. 



41. T. What is an electrophorus ? 



P. A simple 

 electrical appa- 

 ratus, depending 

 upon the princi- 

 ple of induction. 

 It consists of a 

 cake of resin 

 fused in a cir- 

 cular plate of 

 metal which is J'fo. 154 



larger than the resinous disc, and hns a 

 cover with an insulated handle, which is 



* Fiy. 13. Plate-glass electrical machine. A, 

 conductor. B, cushion or rubber, smeared with 



simalgam. c:, c, silk flaps. K, glass plate-, r. 

 winch handle, p, frame-work or pillar. 



t Fiq. 14. End view of the machine, showing 

 the arrangements of parts, a, /', plate ' 

 c, c, c, c, cushions or rubbers, rf, centre oi 

 glass with the axis, <, <, pasting throng* It. /. 

 winch handle, p, p, the frame-work, or pillar. 



J ' t Fi<i. 15. o, b, tho insulated handle, c, rf, the 

 cover, e, disc of shell lac. /, y, conducting 

 j metal disc or sole. 



