34 



ELECTRICITY. 



the complete discharge may be effected 

 as before. 



(122.) By peculiar management a 

 charge may be given to a plate of glass 

 independently of any coating whatever. 

 For this purpose, it must 'be held by 

 one corner, and passed before a ball, 

 connected with the prime conductor of 

 a machine, so that it may successively 

 come in contact with every part of the 

 middle of the plate of glass, while the 

 finger, or any conducting body commu- 

 nicating with the ground, is held oppo- 

 site to it on the other side. Thus the 

 glass will be charged, and will be in the 

 same state as the glass from which the 

 coatings had been removed. 



(123.) We often find, a short time 

 after the discharge of coated glass, that 

 t has acquired spontaneously a small 

 charge, producing a faint spark when a 

 second communication is made between 

 the coatings by the discharging wire. 

 This, which is called the residual charge, 

 arises from two causes : first, a portion 

 of the electricity adheres to the un- 

 coated surface of the glass: and se- 

 condly, another part has penetrated 

 from the coating for some little depth 

 below its surface. Both these portions 

 slowly return to the coatings after they 

 have been deprived of their original 

 charge, and give it a fresh charge. 

 "When a very large extent of coated 

 glass is employed, this residual charge 

 may even amount to a considerable 

 quantity, and the experimenter should 

 be cautious not to expose himself to 

 the shock which he might thus receive, 

 if he inadvertently touched the appa- 

 ratus before he had properly discharged 

 it. That charges are capable of pene- 

 trating even through the entire thick- 

 ness of the glass is proved by the 

 curious fact, that a coated, cylindrical 

 jar may be discharged merely by keep- 

 ing up for a sufficient time a continu- 

 ance of the minute vibrations excited 

 by rubbing it with the finger, or by 

 making it ring. A discharge may also 

 be effected by heating the glass, which 

 renders it a conductor of electricity. 



(124.) The most convenient form for 

 coated glass for experimental purposes, 

 is that of a cylinder or jar. In the ear- 

 lier periods of electrical research, jars 

 were filled with water, mercury, or iron 

 filings, which furnished the interior 

 coating, while the exterior coating was 

 supplied either by water, in which the 

 jar was immersed, or by the hand of 

 the operator, who for that purpose 



grasped the outside of the jar : a rod 'of 

 metal was employed to communicate 

 the charge from the prime conductor of 

 the machine to the inner coating. On 

 making a communication between the 

 exterior and interior coatings, by means 

 of a circuit of conducting substances, 

 the discharge took place, and the shock 

 made to pass through the circuit thus 

 formed. This instrument having been 

 made known principally through the 

 experiments of Kleist, Cuneus, and 

 Muschenbroeck, at Leyden, the name 

 of the Leyden phial, or jar, was gene- 

 rally applied to it. It is at present con- 

 structed as shewn iny?#. 30, by apply- 



Fig. 30. 



ing coatings of tin-foil on both sides of 

 the jar or bottle, leaving a sufficient 

 space uncovered at its upper part to 

 secure it from the risk of a spontaneous 

 discharge, which might take place if 

 the coatings were not separated by a 

 sufficient interval. A metallic rod, rising 

 two or three inches above the jar, and 

 terminating at the top in a brass ball, 

 which is often called the knob of the jar, 

 is made to descend through the cover, 

 till it touches the interior coating. It 

 is through this rod that the charge of 

 electricity is conveyed to the inner coat- 

 ing, while the outer coating is made to 

 communicate with the ground. We 

 have already seen, that if this last con- 

 dition be not observed, the inner coating 

 can receive no charge, and only a feeble 

 spark will pass from the conductor to 

 the knob. 



(125,.) The outer coating may be 

 made to communicate with the ground 

 by holding it in the hand ; and on pre- 

 senting the knob of the jar to the prime 

 conductor when the machine is in mo- 

 tion, a succession of sparks will pass 

 between them, while at the same time 

 nearly an equal quantity of electricity 

 will be passing out from the exterior 

 coating, through the body of the person 



