Chap, i.] LAWS OF ELECTRICAL ATTRACTION. 5 



A bad conductor, when united in this way to a good con- 

 ductor to prevent the escape of the electricity, is called 

 an INSULATOR. The best conductors are the metals, 

 and following them are carbon, plumbago, acids, saline 

 solutions, animal fluids, water, animal and vegetable 

 tissues, and moist stones and earth. The best insulators 

 (bad or non-conductors) are shellac, amber, resins, wax, 

 glass, ebonite, guttapercha, silk, wool, feathers, porce- 

 lain, paper, oils, dry air, and wood. The human body is 

 a good conductor, dry air a bad one. It is difficult to 

 perform electrical experiments in an atmosphere con- 

 taining aqueous vapour, because a film of moisture is 

 deposited on the insulating supports of the apparatus, 

 rendering the insulation imperfect : hence the benefit in 

 damp weather of heating the apparatus just before use. 



It is also in virtue of the fluid which it contains 

 that the human body is a conductor, water being a 

 good conductor. A charged body in a current of air 

 slowly loses its electricity by convection. Particles of 

 the air coming in contact with the body receive a 

 charge, and pass on, to be succeeded by other particles, 

 each of which also carries off its portion, till the whole 

 charge is thus dissipated. 



The laws of electrical attraction and re- 

 pulsion are as follows : 



(1) Like electricities repel one another. 



(2) Unlike electricities attract one another. 



(3) The force of attraction or repulsion varies 

 inversely as the square of the distance between the 

 two electrified bodies, and directly as the amount of 

 charge of the two bodies. 



Electricity accumulated solely on tlie sur- 

 face of conductors. If the body have a spherical 

 surface, the electrical layer is equal at all points of 

 the surface ; that is, is of uniform DENSITY, density 

 being the thickness of the layer of electricity. If, 

 however, the conductor be an ellipsoid, the electricity 



