170 



UNDl'LATOBY FORCER ELECTRIC I TV 



[ELECTRICAL MA 



amber, and a sheet of gutta-percha, The 

 will bo produced. 



gmm,nm>*l i Dry a sheet of paper before a fire 



ntiian moisture fa rMnoved from it, and then place it flat 



. . . i: .tbriatytn m m -:- f 



India rubber, and then suddenly lift it fro,,, the table 



IBS It w,ll adhere thereto . l-ut on bring removed, 

 and held vwtioally by the hand, it will approach to the 

 fingers, if they are held near to it ; and if the cxperi- 

 Mat be tried in a dark room, a small spark will be 

 seta to pas* when the fingers and the paper are nearly 



ntaet. This experiment shows both an instance of 

 |H*M| *rorh<m and the production of tbe lUttrie 

 . . 



flhpm'mmf 4. The fur of animals is also capable of 

 affording considerable electric-] effects. A piece of pre- 

 paredskin may be rubbed, on the fur side, by means of 

 dry silk handkerchief. But the beet plan is to rub 

 the fur of a black cat, from tho head towards tho tail, 

 either by means of the dry hand, or by a silk handker- 

 chief. It will be found that each hair separatee from 

 ita neighbour giving an illustration of what is called 

 sfsrji'ii'iiif rrjntliion. And if the experiment be tried in 

 a dark place, abundant sparks may be obtained by pre- 

 senting the fingers to the fur ; a sharp crackling sound 

 being noticed at the same time. 



The above will afford a sufficient variety of illustra- 

 tions, showing how friction disturbs the natural elec- 

 trical state of hoilies ; and an elementary idea will also 

 be gained of what U meant by the terms "attraction" 

 aod"repnlsion," in connexion with electricity ; on which 

 we shall treat more fully hereafter, as our subject ex- 

 pand* to our view. 



ELECTRICAL MACHINES. 



W now proceed to describe a variety of arrangements, 

 by mosns of which we obtain a considerable amount of 

 free electricity. We consider this as the best course, 

 because the student must be provided with some instru- 

 ment of the kind before he can perform any experi- 

 ments elucidating the law of the science. We shall 

 describe those in ordinary use, and afterwards suggest 

 simpler and cheaper arrangements, which any person, 

 with an ordinary mechanical turn of mind, may, at a 

 alight cost, construct for his own use. 



Glass is universally employed in the usual form of 

 electrical machines ; and these are of two classes the 

 cylindrical and the plate. Each kind of machine has 

 its advocate ; but for all practical purposes, we recom- 

 mend the use of the plate machine. 



The cylindrical machine consists of a glass cylinder, 

 mounted on two upright supports. On one side a rubber, 

 which, by its friction, produces electricity, is supj>orted 

 on a glass rod, so at to press firmly on the surface of tho 

 (lass. On the opposite side of the cylinder, a ball or 

 cylinder of brass is placed ; its surface, next to tlic 

 evliiider. sing coated with metal points. This U called 

 the conductor, and, like the rubber, is also mounted on 

 Bias* legs. We may here, without entering at present 

 Into the laws of conduction, observe, that glass, resin, 

 gutta-percha, *c., convey electricity very slowly over 

 their surface ; and- hence have been termed non-con- 

 ductors. Now, as it is requisite that tho force in 

 by tbe maririne should be guided in one particular din-c 

 .0 "conductor" itself is formed of a good con- 

 ducting substance, whirh is always a metal ; whiUt it and 

 the rubber are insulated electrically by means of tin- 

 glass rods on which they stand. At the end of tbe con- 

 ductor a brass knob fa placed, from which the spark is 

 received. Points being useful in collecting electricity, 

 are, therefore, as we have stated, exposed to the 

 glass surface ; whilst the braes knob, which has a kind ..f 

 oonservine (mwi-r. forms the terminal of the conductor 

 at that iN.int ftirthent from tho machine. Tin- cylinder 

 i* ttinird, by means of a handle, in tho direction of from 

 th<- rubber towards the conductor. The follow 

 graving illustrates the various parts of a cylindrical 

 u, i, ' M 



i i. 



In the above diarratn. a I repreaent tb* g\am cylinder ; e t, the lei by 

 which it U tupponed ; t J, the winch handle, by which the cylinder u 

 turned; a. the rubber; h, tbe conductor; i, the (rlo>* cupport to the 

 rubber ; it, the irUvi cupport to the conductor ; m n. a movuble foot, by 

 which tbe dutance between the rubber and the cylinder can be ad- 

 jnited. Thew arranireinenti rary lithtly. according to the riewt of 

 the maker ; but the different parti an common to erery form of the 

 machine. 



As each kind of glass machine is similarly prepared for 

 experimental pur|>oses, we shall drfrr i-ntoring into such 

 particulars till aftor the description of the plate machine. 

 These have been made in various forms. Occasionally 

 two rubbers are used, which are placed at the top and 

 bottom of the plate. In another form, the rubber is 

 placed on one side, and the conductor on the opposite 

 side, of the arrangement. Mr. Woodward recommended 

 the use of two plates and two seta of rubbers, etc. : this 

 arrangement is but very rarely seen at tho present day. 

 Generally speaking, small plate, machines are fitted with 

 double rubbers, because they can easily be adjusted; 

 whilst those of largo size have only one rubber. This 

 is a question which may be left to the views and decision 

 of tho experimenter; bearing, however, in mind, that n 

 one-rubber machine can bo turned more rapidly, and, 

 therefore, by this means affords scarcely less electricity 

 than those of a similar size fitted with double rubbers. 

 In our opinion, there is leas risk of fracture ari-ing from 

 unequal tension of the niblwrs on the HJ indie or axis of 

 the plate, in a single-rubber machine. 



The following engraving illustrates a double-rubber 

 plate machine. 



Fig. 1. , 

 



it I* tbe tliM plat*; t ft, are tbe rubbers, fitted by elampa to an up- 



riehl frame ; r l a flap of black ailk. which cm era the gfaM plate ; d 



if, vtturh extends, by means of two arm* roverert with 



points, in tlir !. - :,',n of the plate; they terminate at the endiof the 



ilk Bapa, and o eolu . idled glass surface ; 



r i. tin- hrnM kn..b of tlir conduct >r; //, are iu two R!;IM insulating 



upporU; p ! the hnnulc, by mem* of which the plati- i* turned bt?- 



rubber" ; A i tho wooden table, on bich the different parU 



of tbe machine art tied. 



