6, 7] The Fundamental Conceptions of Electrostatics 7 



tolerably well, and hence a wet body is generally a bad insulator. So also an 

 electrified body suspended in air loses its electrification much more rapidly in 

 damp weather than in dry, owing to conduction by water-particles in the air. 



When the body is in contact with insulators only, it is said to be 

 " insulated." The insulation is said to be good when the electrified body 

 retains its electrification for a long interval of time, and is said to be poor 

 when the electrification disappears rapidly. Good insulation will enable a 

 body to retain most of its electrification for some days, while with poor insula- 

 tion the electrification will last only for a few minutes or seconds. 



III. Quantity of Electricity. 



7. We pass next to the conception of a definite quantity of electricity, 

 this quantity measuring the degree of electrification of the body with which 

 it is associated. It is found that the quantity of electricity associated with 

 any body remains constant except in so far as it is conducted away by con- 

 ductors. To illustrate, and to some extent to prove this law, we may use 

 an instrument known as the gold-leaf electroscope. This consists of a glass 

 vessel, through the top of which a metal rod is passed, supporting at its lower 

 end two gold-leaves which under normal conditions hang flat side by side, 

 touching one another throughout their length. When an electrified body 

 touches or is brought near to the brass rod, the two gold-leaves are seen to 

 separate, for reasons which will become clear later (21), so that the instru- 

 ment can be used to examine whether or not a body is electrified. 



Let us fix a metal vessel on the top of the brass rod, the vessel being 

 closed but having a lid through which bodies can be in- 

 serted. The lid must be supplied with an insulating 

 handle for its manipulation. Suppose that we have 

 electrified some piece of matter to make the picture 

 definite, suppose that we have electrified a small brass 

 rod by rubbing it on silk and let us suspend this body 

 inside the vessel by an insulating thread in such a 

 manner that it does not touch the sides of the vessel. 

 Let us close the lid of the vessel, so that the vessel 

 entirely surrounds the electrified body, and note the 

 amount of separation of the gold-leaves of the electro- 

 scope. Let us try the experiment any number of times, 

 placing the electrified body in different positions inside 

 the closed vessel, taking care only that it does not come 

 into contact with the sides of the vessel or with any FlG - * 



other conductors. We shall find that in every case the separation of the 

 gold- leaves is exactly the same. 



