ELECTRICITY. 



39 



beam, upon knife-edged centres. One 

 of the arms of this beam is graduated, 

 and carries a slider, which, when set 

 at different distances from the centre of 

 motion, acts on the lever with a pro- 

 portionate weight from one grain to 

 sixty. The ball A, at the extremity of 

 this loaded arm, rests on a similar ball 

 D, below it, which is supported by a 

 bent metallic tube T, proceeding from 

 the same stand as that which supports 

 the rods ; the whole being insulated by 

 a glass pillar P. At a little distance 

 below the ball B, at the other extre- 

 mity of the beam, another ball C, insu- 

 lated by the glass pillar Q, is placed; 

 this last ball is to be connected by a 

 chain with the outer coatings of the 

 battery, while the metallic support of 

 the balance is connected with the inner 

 coatings. When a charge is commu- 

 nicated to the battery, the two balls A 

 and D, which are in contact, become 

 repulsive of each other ; and when the 

 force of this repulsion is sufficient to 

 raise the weight on the loaded arm of 

 the beam, the other arm will be forced 

 down, and the ball B coming in con- 

 tact with the ball C, the circuit will be 

 completed and a discharge take place. 

 As the force of the repulsion depends 

 upon the intensity of the charge, the 

 weight it has to overcome affords a 

 measure of this intensity, and enables us 

 to regulate its amount. 



The practical application of accumu- 

 lated electricity to various purposes of 

 experiment, involves considerations 

 which relate to the laws observed by 

 electricity in its movements, and which 

 more properly belong to the subject of 

 the ensuing chapter. 



CHAPTER X. 



Of the Motion of accumulated Elec- 

 tricity. 



(140.) IN forming arrangements for 

 directing the passage of accumulated 

 electricity, it should be borne in mind 

 that the electric fluid will, on these oc- 

 casions, always pass through the best 

 conductors, although they may be more 

 circuitous, in preference to those which 

 are more direct, but have inferior con- 

 ducting power: and it must also be 

 recollected, that when different paths 

 are open for its passage, along con- 

 ductors of equal power, the electricity 

 will always take that which is the 

 shortest. Thus if a person, holding 

 a .wire between his hands, discharges a 



jar by means of it, the whole of the 

 fluid will pass though the wire, without 

 affecting him : but if a piece of dry 

 wood be substituted for the wire, he 

 will feel a shock ; for the wood, being 

 a worse conductor than his own body, 

 the charge will pass" through the latter, 

 as being the easiest, although the 

 longest circuit. During its transit 

 through the human body, in like man- 

 ner, the shock is felt only in the parts 

 situated in the direct line of communi- 

 cation ; and if the charge be made to 

 pass through a number of persons who 

 take one another by the hand, and form 

 part of the circuit between the inner 

 and outer coatings of the jar, each will 

 feel the electric shock in the same 

 manner and at the same instant ; the 

 sensation reaching from hand to hand, 

 directly across the breast. By varying 

 the points of contact, however, the 

 shock may be made to pass in other 

 directions, and may either be confined 

 to a small part of a limb, or be made 

 to traverse the whole length of the 

 body from head to foot. 



(141.) By accurate experiments it ap- 

 pears that the force of the electric shock 

 is weakened, that is, its effects are di- 

 minished, by employing a conductor of 

 great length for making the discharge. 

 But it is difficult to assign a limit to the 

 number of persons through which even 

 a small charge of electricity may be 

 sent, so that all shall experience the 

 shock ; or to the distance along which 

 it may be conveyed by good conductors. 

 At an early period of electrical inquiries, 

 much interest was attached to the de- 

 termination of these points. The Abbe 

 Nollet passed an electrical shock from 

 a small phial through a hundred and 

 eighty of the French guards in the pre- 

 sence of the king ; and at the Carthusian 

 convent in Paris, the monks were formed 

 into a line of above a mile in length, by 

 means of iron wires held between them : 

 on the discharge of the phial, the sensa- 

 tion was felt at the same moment by all 

 the persons composing this extensive 

 circuit. Many experiments were made 

 both by the English and French 'elec- 

 tricians with a view to ascertain the 

 space which a discharge can be made 

 to traverse, and the velocity with which 

 it is transmitted. Of these the most 

 ingenious and satisfactory were the ex- 

 periments planned and executed by Dr. 

 Watson, with the assistance of the lead- 

 ing members of the Royal Society. A 

 circuit was formed by a wire which ex- 



