SiO 



LESSOXS IN ELECTRICITY. 



PIG. 26. 



excited by friction, and the * ' prime con 

 ductor." 



The sulphur sphere of Otto von Guc- 

 rickc was, as already stated, the first 

 electrical machine. The hand was the 

 rubber, and indeed it long continued to 

 be so. For the sulphur sphere, Ilauks- 

 bcc and Vv'incLlcT substituted globes cf 

 glass. Doze of Wittenberg (1741) add- 

 ed the prime conductor, which was at f:r t 

 a tin tube supported by resin, orouspcnu- 

 ed by sill:. Soon afterward Gordon 

 substituted a glass cylinder for the globe. 

 It "was sometimes mounted vertically, 

 sometimes horizontally. Gordon so in- 

 tensified l.i.'i discharges as to be able to 

 kill email birds with them. In 17CO 

 Planta introduced the plate machine now 

 commonly in r.se. 



Mr. Cottrell has constructed for these 

 Lessons the email cylinder machine shown 

 in fig. 20. The glass cylinder is about 7 

 inches long and 4 inches in diameter ; 

 its cost is eighteen pence. Through the 

 cylinder passes tightly, as an axis, a 

 pidcc of lath, rendered secure by sealing- 

 wax where it enters and where it quits 

 the cylinder. G is a glass rod supporting 

 the conductor c, which is a piece of lath 

 coated with tin-foil. Into the lath is 

 driven the scries of pin points, r, r. The 

 rubber n, is cccn at the further side cf 

 the cylinder, supported by the upright 

 lath r/, and caused to press against the 

 glass. G' ha flap of u'lk attached to the 

 rubber. Y/hen the handle 13 turned 



FIG. 27. 



sparks may be taken, or a Leyden jar 

 charged at the knob c. 



A plato machine h thown i:i fig. 27. 

 p b the plate, which t;:rn3 on an axi.i 

 passing through its centre ; r. and r/ arc 

 two rubbers which clasp tho plate, with 

 the flips of cilk G G' attached to them. A 

 and A' arc rows of points forming part cf 

 the prime conductor, c. G G' is an insu- 

 lating rod cf glass, which cuts off tho 

 connection between the conductor and 

 the handle of the machine. 



The prime conductor is charged in the 

 following manner. When the glass plato 

 is turned, as it passes each rubber it is 

 positively electrified. Facing tho elec- 

 trified glass is the row of points, placed 

 midway between the two rubbers. On 

 these points the glass acts by induction, 

 attracting the negative and repelling the 

 positive. In accordance with the princi- 

 ples already explained in 10, tho nega- 

 tive electricity streams from tho points 

 against the excited glass, which then 

 passes on neutralized to the next rubber, 

 where it is again excited. 



Thus the prime conductor is charged, 

 not by tho direct communication to it of 

 positive electricity, but by depriving it 

 of its negative. 



If when tho conductor is charged you 

 bring tho knuckle near it, the electricity 

 passes from the conductor to tho knuckle 

 in the form of a spark. 



Take this spark with the blunt knuckle 

 while the machine is being turned ; and 



