438 



ELECTRICITY- 



rcxi.ui. 

 r>g. IT. 



Effect of 

 points 

 on electric 

 light 



BrilliaUcy 

 and length 

 of the 

 spark* 



EBectiof 

 the medium 

 through 

 which the 

 (park ii 

 transmitted. 



if no ball were present. Mr Nicholson, to whom we 

 ; are indebted for the preceding experiments, remarks, 

 " that the effect of a positive surface appears to extend 

 further than those of a negative one, for the point act* 

 like a ball when considerably more prominent, if it be 

 positive, than it will if negative." 



Mr Nicholson has applied the preceding principle 

 to the construction of a simple instrument for distin- 

 guishing positive from negative electricity, when they 

 are sufficiently strong to afford sparks. Two metallic- 

 balls A, B, Fig. 17. may be placed at different distan- 

 ce*, by means of a joint at C. The arms CA, CB, are 

 made of glass, and covered with varnish, and a short 

 point /) projects from the ball B towards the ball A. 

 This simple apparatus is now to be placed near any 

 electrified body, so that the electric jxwer may pass 

 through the balls ; then if the electricity passes from A 

 to B, there will be a certain distance of the balls, at 

 which a spark will pass between them, and this dis- 

 tance will be much shorter when the electricity is pass- 

 ing from B to A. 



171. 7. Hold an uninsulated sheet of paper near a 

 positively electrified conductor, and the electrical light 

 v. ill form a beautiful star, about four inches in diame- 

 ter, consisting of distinct radii, but not ramified. Per- 

 form the same experiment with a negatively electrified 

 conductor, and instead of forming a star, it will throw 

 out many pointed brushes of light to the paper. 



Exp. 8. Present the point of a needle to a body posi- 

 tively electrified, in the dark, and the point will be il- 

 luminated with a star ; but when the body is negative- 

 ly electrified, it will exhibit a pencil or brush of light. 



From the two preceding experiments, it becomes very 

 easy to distinguish positive from negative electricity, 

 by means of the different characters of the electric light 



Exp. 9- It appeared from experiment 6, that the ef- 

 fect of a point was destroyed, when it was withdrawn 

 into a large brass ball, and sparks were produced as if 

 the point had been wholly removed. The same effect 

 will be obtained if the point is inclosed in a glass tube, 

 or if it is placed between two balls. 



Exp. 10. When sparks are passing between the con- 

 ductor and a brass ball 1 ^ inches distant, present a 

 sharp point at double that distance, and the sparks will 

 no longer appear. 



The brilliancy of the electrical spark is proportional 

 to the conducting power of the bodies between which 

 it passes. Hence metals are used for the production of 

 the electric spark. When an imperfect conductor, such 

 as wood, is employed, the electric light appears in the 

 form of faint red streams. The length of the spark 

 depends upon the power of the electrical machine, and 

 the state of the weather when the experiment is made. 

 With a very powerful electrical machine sparks may 

 sometimes be procured from 10 to 20 inches long, and 

 these sparks always pass from die positive to the nega- 

 tive ball. * 



The electric spark always varies its character with 

 the nature and density of the medium through which 

 it is transmitted. The earliest experiment upon this 

 subject was made by Dr Watson, on a large scale, and 

 is one of the most beautiful which can be exhibited. 



Exp. I. Take a glass cylinder, three feet long and 

 three inches in diameter, and fit it up at one end so as to 

 be able to let down a brass plate from the top, in order 

 to approach another brass plate fixed at the bottom of 

 the cylinder. Let the cylinder be now exhausted and 



insulated. When tlie upper plate is electrified, the clec- Tt<- 

 trie matter will jwiss from one plate to another, at the K1> 

 greatest distance at which the plates can !>< placed, and s ~ 

 the lower plate will be electrified as if it had been con- 

 nected with the prime conductor. When the room in 

 dark, the electric matter, which in the open air pa- 

 small brushes or pencils of light about an inch or two 

 long, will now be transmitted in vivid coruscations ( ,t" 

 a bright silver hue along the whole length of the tube. 

 Thc-e coruscations do not divide as in the open air, 

 but from a base apparently Hat, they frequently divide 

 themselves into .-.mailer and smaller ramifications, like 

 the coruscations of the northern lights. When the va- 

 cuum is very perfect, the electric matter will pass l>e- 

 t \\.-i n the plates in a continued stream of the ?anu 

 throughout its whole length 



y-.'jy). 2. Having placed the brass plates at the dis- 

 tance of 10 inches, make this vacuum a part of the cir- 

 cuit through which a jar is discharged, and at the in- 

 stant of the discharge, a mass of bright embodied fire will 

 be seen to leap from one of the brass plates in the tube 

 to the other. When the distance of the plates is great- 

 er than 10 inches, the light begins to diverge and lim- 

 its force, and this divergency is nearly proportional to 

 the distance of the plates. 



F.XJI. 3. Fonn a Toricellian vacuum in the angular 

 part of a long bent tube of glass filled with mercury 

 and inverted, and place the legs of the bent tube in .se- 

 parate basons of mercury. If this vacuum is insulated, 

 and one of the basons of mercury made to communicate 

 with the conductor, while some non-electric s-ubstance 

 touches the other, the electric light will pervade the va- 

 cuum in a continued arch of lambent flame, without the 

 least divergency. If one of the basons be connected 

 with the machine, which should be insulated, the light 

 will pervade the vacuum in a contrary direction. 



These three experiments were made by Dr Watson. 



Exp. 8. Through the top of the receiver of an air- 

 pump, a foot high and half a foot broad, insert a point- 

 ed wire, so as to project two inches into the inside. 

 Place the receiver on the plate of the air-pump, and con- 

 nect the wire with the prime conductor of the electrify- 

 ing machine. A brush of light will appear at the point 

 of the wire ; but as soon as the exhaustion of the recei- 

 ver commences, this brush will enlarge itself, and vary 

 its shape ; and when the rarefaction is considerable, the 

 whole of the receiver will be filled with a beautiful blush 

 of light, the colour of which changes with the strength 

 of the electricity. 



Exp. 9- Into the extremity of a glass tube 30 inches, 

 insert an iron wire projecting into the tube, and having 

 a ball on the outer end of it. Fill the tube with mer- 

 cury, and at the open end place a drop of ether, and 

 secure it by the finger, while the tube is inverted so as to 

 form a Toricellian vacuum in the upper part : The ether 

 will rise to the top, and upon the removal of the finger, 

 and the descent of the mercury, it will expand into va- 

 pour. If electricity is now transmitted through this 

 vapour it will become luminous, and assume various 

 hues according to its strength. If a strong spark passes 

 through some inches of the vapour, the light is general- 

 ly of a blue colour. 



In condensed air and in carbonic acid gas, the light 

 of the spark is white and brilliant, but in hydrogen gas 

 it is red and faint. 



Exp. 10. Charge a Leyden phial by its knob, and 

 take away the knob by a stick of sealing wax. Place 



Sipgen' EUmenU of tectricity, p. 61. 



