220 MR. HARRIS ON SOME ELEMENTARY LAWS OF ELECTRICITY. 



The approximations observable in this Table to the law in question are as perfect 

 as can be expected, which is further evident when given quantities are transferred to 

 a plane conductor in contact with the electrometer a, fig. 2, as will be more fully ex- 

 plained hereafter (56.). 



18. Similar results may also be arrived at in disposing given quantities of electri- 

 city on coated jars. By aid of the unit jar (11.), and an improved adaptation of the 

 common balance as a measure of electrical attraction, they can be exhibited without 

 the least difficulty. It becomes necessary that I should briefly mention here the more 

 recent mechanical arrangements connected with these and similar experiments, as I 

 shall have occasion to refer to them frequently. 



19. Fig. 9. N. represents a simple balance, suspended from the curved brass rod 

 n h. It can be raised or depressed through small distances by a micrometer-screw 

 at h, and can be also elevated or depressed by the graduated sliding tube n o : the 

 tube o is screwed on a brass cap, fixed on the glass column N, through the centre of 

 which passes a stout brass wire. A conducting substance m, of any required form, is 

 suspended by a double silver thread from one of the arms of the beam : it is made of 

 light wood, is hollow, and gilded. This body is accurately counterpoised by weights 

 placed in the scale-pan t, suspended from the opposite arm. A similar conductor nC 

 is fixed immediately under the former, and is supported on a graduated sliding tube s, 

 insulated on the glass pillar h : the pan t, when loaded with given weights, rests on a 

 small plate of wood, whose altitude can be easily adjusted by means of the sliding 

 brass rod r : the whole is fixed on an elliptical base, furnished with three levelling 

 screws. 



When the lower conductor ni is connected with one side of an electrical jar E, 

 through the substance of the ball b, and the suspended conductor m with the opposite 

 side, by means r>f the suspension thread and the wire passing through the glass 

 column N, then the attractive force arising from a given accumulation is caused to 

 act immediately between these conductors m rd, and may be measured, under given 

 conditions, by weights placed in the pan t. 



The distance between the nearest points of the conductors m m" is accurately esti- 

 mated in the following way : The insulated conductor rri being raised to zero of the 

 graduated tube, so as to touch, or very nearly so, the suspended body m, the points 

 of contact are minutely found by the micrometer screw h. The body m' is now de- 

 pressed a given quantity, as measured by the divisions on the slide, and hence the 

 distance between m rd is accurately known. When this distance requires to be greatly 

 increased, it is eflfected by raising the beam, which is easily done by means of the 

 graduated slide n o ; but in effecting this it is essential to raise at the same time the 

 pan t, so as to preserve the index rod of the beam exactly vertical. 



20. These conditions understood, the following experiments will be easily appre- 

 hended : 



{(1.) A jar, E, fig. 9, exposing about five square feet of coating, being connected 



