176 



UNDULATORY FORCES. ELECTRICITY. 



L CONDUCTION. 



of repulsion, is that of attaching a number of narrow 

 tripa of tissue-paper to t!., conductor of tho machine. 

 \\ iu'ii electrified, they will all separate, M in tho two 



l.i>t , \IMTMII, :.t~ 



Electroscope* have been constructed, which are in- 

 tended to show the presence of free electricity. They 

 are chiefly of two kind* the pith ball and gold-leaf 

 electrometers, as they are sometime* alao called. The 

 .or alone deserves that name, 

 V j because it registers, by the extent 

 of repulsion indicated on a gra- 

 duated quadrant, tho amount of 

 free electricity present. Its con- 

 struction is as follows. (See Fig. 15). 

 Now, when this electromotor is 

 placed in contact with a non-elec- 

 trised body, the ball, a, will rest 

 in contact with / ; but if the body 

 be electrified, then a will be re- 

 pelled from /, and will rise towards 

 f the top of the quadrant, just in pro- 

 portion to tho amount of free elec- 

 tricity present : the number of de- 

 grees marked on the qu;idrant, by 

 means of the straw attached to the 

 i. pith biuupendc<i pith ball, of course indicates the 

 * of electrisation. 



Such an instrument, whilst acting 

 tolerably well when a large amount 

 of free electricity can be observed, 

 jg o f f a r too coarse a nature for 

 detecting minute electrical disturbances ; and, for 

 this purpose, the gold-leaf electroscope is of great value. 



upright >und, t; el* 



u> SS^Tu'aS 

 tuchtd to the up- 

 right und. 



rig. 10. 



One of these is represented in Fig. 16 : 

 in which A is a brass cap, fitting on a 

 cylindrical glass vessel, B ; C C are two 

 pieces of gold-leaf, suspended from tin- 

 brass cap ; D D are pieces of tinfoil, 

 |B pasted on the opposite sides of the glass 

 cylinder. Now, so long as the brass cap 

 is not electrified, the gold leaves, C C, 

 hang together in contact ; but on pre- 

 1 sonting any electrified body to A, the 

 leaves immediately repel each other, 

 even although the free electricity is ex- 

 tremely slight in quantity. We should, 

 however, state, that the indications of 

 this instrument entirely depend on the state of the 

 glass cylinder. If this is in the least dirty or moist, 

 the electricity at once passes away. This electro 

 must therefore be carefully dried before use, and should 

 be slightly elevated in temperature beyond that of the 

 surrounding air. 



An instrument of this kind is easily made by em- 

 ploying an ordinary gas chimney-glass. In the inside, 

 to about half its length, two narrow strips of tinfoil 

 are to be pasted opposite to each other ; the bottom of 

 the glass should then be closed with a cork. Into the 

 top, another cork, entirely covered with tinfoil, may be 

 fitted ; and to the inside of this, two strips of gold-leaf 

 are to be attached, so that they may hang inside the 

 glass with tho lower end a little below the top of the 

 tinfoil strips, as shown in Fig. 16. Such an instru- 

 ment, if carefully made, will answer every purpose 

 winch can be required for many delicate electrical 

 experiments. 



Various forms of electroscopes and electrometers have 

 been devised, each being suited to some special purpose. 

 Amongst the most delicate of these is that invented by 

 Professor Thomson, of Glasgow ; which was used during 

 the laying of the Atlantic telegraph. Its peculiar con- 

 struction prevents the possibility of illustrating it by an 

 engraving : we may observe, however, that its indica- 

 tions of the feeblest electrical disturbance, makes it of 

 great value in many investigations. 



Generally speaking, electrical attraction and i-'-]ml- 

 sion are coincident. The reason of this we shall under- 

 stand better when we examine into the opposite elec- 

 trified conditions which bodies assume, a:;ii which have 



Fig. 17. 



1111 



been termed ixiaitive and negative. For the present, 

 we shall only illustrate the ] IK n<>nu n.i. without filtering 

 deeply into their causes ; and adduce homo experiments 

 illustrative of attraction. 



If a metal plate bo 

 hung from the coii.li, 

 of tho electrical machim , 

 and some pith balls be 

 placed within two or 

 three inches of it, they 

 will be attracted towards 

 the plate. An amusing 

 inixle of showing this is 

 illustrated in Fig. 17 ; 

 in which we observe a 

 stand for holding figures 

 of men. Arc., cut out of 

 pith, for the sake of 

 lightness. At a distance 

 of three or more inches, 

 tin plate from the con- 

 ductor is suspended. On 

 working the machine, 

 the figures will commence dancing between the plates, 

 owing to the electrical attraction and repulsion which 

 ensues. 



Another mode of showing the same effects, is that 



of the electrical bells. The arrangement consists of 



three bells and two clappers, suspended from a beam, 



which is to be attached to the conductor of the machine. 



Fig. 18. The bells are electrified 



O whilst the clappers oscillate 



between them ; and so long 

 as the machine in worked, a 

 continual ringing of the bells 

 tV...'- place. A chain is hung 

 from the centre bell, so as tu 

 communicate, with tin 

 The ordinary form of this 

 apparatus is shown in Fig. 

 18. 



Electrical see-saws, <fcc. , 

 are constructed by the in- 

 strument-makers, for illus- 

 trating the general laws of 

 attraction and repulsion, 

 i Tho intensity of these 



forces varies inversely as the square of the distance 

 between the electrified Ixxly and that on which the at- 

 tractive or repulsive effect takes place ; and this law 

 agrees with that governing all radiating forces, as wo 

 have previously explained in our sections on heat and 

 light. There are, however, so many interfering causes 

 which disturb the exactitude of an experimental illus- 

 tration of this fact, so far as electricity is concerned, 

 that whilst, as an inference, we may entirely trust to 

 its truth, we have not such rigidly-founded premises 

 and conclusions as we can obtain when investigating the 

 other undulatory forces. 



CONDUCTION. 



WE have already stated, that all bodies have, electrically 

 speaking, been divided into two classes namely, con- 

 ductors and non-conductors ; or, what is equivalent 

 thereto, electrics and non-electrics. This distinction, 

 however, has been made without any title to correct- 

 ness ; for we are acquainted with no body which will 

 not, to some extent, however trifling, conduct electri- 

 city ; and, on the other hand, we can never call any 

 body incapable of affording evidences of free electricity, 

 when proper means are employed to that end. \\ e 

 shall therefore use such terms with the limitations we 

 liave named, and as expressions which, whilst they 

 assist to convey definite ideas to the minds of our 

 readers, are not to be considered as incorporating the 

 real facts of the case. 



By the term "conduction," we mean that ability 

 which some substances have, of exhibiting, at a distance 



