ELECTRICITY. 



313 



Tn oil those tho shook is given w soon aa the necessary charge 



is communicated ; this in Komutimuu rather a disadvantage, :wid 



may bo obviated by the use of a unit jar, which consist* of a 



hmall l.i'\tl.-ii j.-ir exposing about nix inches of coated surfaoo on 



1<>. This is supported on au insulating stand, so that its 



r is in contact with tho knob of the jar to bo charged. 



and ball are also connected with its outside coating, and 



! within striking distance of its inner rod. The jar is 



;i; ht to the conductor of the machine, and as it becomes 



tilled u corresponding amount of electricity passes from its 



r into tho large jar. This continues till the unit jar is 



fully churned, wlim it 



irges itself, and s^. **. 



tho same process is re- 

 peated. In this way, 

 by counting tho num. 

 ber of discharge* of 

 the small jar, wo as- 

 in the number of 

 unit s of electricity con- 

 1 in tho large 

 one. 



As we now clearly 

 understand the con- 

 struction and action of 

 the Leyden jar and 

 battery, we may notice 

 some of the effects pro- 

 duced by the electric 

 shock. We will look 

 at the physiological 

 effects ; and the first 

 which we observe is 

 the peculiar sensation 

 experienced when the 

 charge is allowed to 

 pass through the body. 

 The sensation varies, 

 of course, with the in- 

 tensity of the shock, 

 and is most strongly 

 felt at tho elbows and 

 across tho breast. If 

 the shock of a large 

 battery be taken dan- 

 gerous results may 

 ensue, especially if 

 the electric fluid pass 

 through any vital por- 

 tion ; but with ordi- 

 nary charges no ill 

 effects whatever seem 

 to bo produced ; in 

 some cases it even ap- 

 pears to be beneficial. 

 The effect of a very 

 strong charge is seen 

 when a person is struck 

 by lightning, death 

 being frequently 

 caused by it. The ac- 

 tion appears to bo 



mainly en the nervous system. If we pass the shock through 

 small animals, such as birds or mice, they will be killed instan- 

 taneously, and larger animals have been killed by shocks from 

 powerful batteries. There is, however, nothing further to be 

 learnt by those experiments, and it is therefore cruel and need- 

 less to repeat them. 



The electric fluid, in its passage through living or dead bodies, 

 also causes convulsive contractions of tho muscles. Its physio- 

 logical effects will, however, be seen much more clearly when wo 

 come to treat of Voltaic Electricity, and wo may therefore leave 

 further notice of the matter till then 



In studying the effects produced by electricity, we need some 

 method of conveniently holding an object while the shock is 

 passed through it, and this is afforded us by a very useful piece 

 of apparatus, known as Henley's Universal Discharger. The 

 construction and mode of nsing this will be understood by 



reference to Fig. 19, which represent* the charge from a battery 

 being pawed through a bird by mean* of this discharger. 



A piece of hard wood is taken, about twenty inch** long 

 and six inches wide, and in the middle of it then i* fixed a 

 small table, which can be adjusted at a convenient height by 

 means of a thumb-screw. The top of this table i* usually made 

 of a duo of glass fixed on by shellac, or else it has a strip of 

 ivory inlaid across it At each end of the board U fixed a glass 

 rod, carrying at the top a revolving cop fitted with a compass 

 joint, so that the wire passing through it may be inclined at any 

 angle. The wires can- also be slipped backwards or forwards, so 



that they may easily 

 be brought into con- 

 tact with any part of 

 the substance to be 

 operated upon. These 

 wires are pointed at 

 the ends, and brass 

 balls are fitted so as 

 to screw on over the 

 points when required ; 

 but as brass balls are 

 rather expensive, lead 

 bullets may, in most 

 cases, be substituted 

 for them, the main dif- 

 ference being in the 

 appearance. One of 

 these instruments will 

 be found to be of great 

 use to the student, and 

 he should therefore en- 

 deavour to procure, cr, 

 better still, to make> 

 one; for a better know- 

 ledge of the principles 

 of a science may usu- 

 ally be obtained by 

 makingapparatusthaa. 

 by merely experiment- 

 ing with that already 

 made. As shown in 

 the figure, the object 

 through which the 

 charge is to be passed 

 is placed on the table, 

 and the knobs or points 

 of the wires brought 

 into contact with oppo- 

 site sides of it One 

 wire is then connected 

 with the outside of the 

 jar or battery, and the 

 other connected with 

 the knob by means of 

 the jointed discharg- 

 ing rod, contact being, 

 mode with the wire of 

 the discharger first 

 For this purpose, how- 

 ever, it is necessary 



that it should have an 



! insulating handle, as the electricity always chooses the best 



j conductors, and if there be any distance between the knobs, or 



' if a badly conducting substance be interposed, the electricity may 



travel through the body of tho operator to the ground instead 



of passing through the discharger 



The next class of effects wo notice are the luminous. Aa 

 already seen, whenever tho spark traverses air a line of light 

 is pcen ; when the two electricities from the opposite sides of 

 a jar unite, tho spark is much more luminous, being thicker 

 and denser, because of the passage of a much greater amount 

 of the fluid. This spark is almost instantaneous, so that if a 

 dark room bo suddenly illuminated by it, a rapidly revolving 

 wheel in it will appear to be at rest. So instantaneous is it, 

 that a printed bill has been fixed on to a wheel, and made to 

 revolve several hundred times per nv'~ >, and yet a distinct 

 photograph of it has been taken by .. .... of the light from 



