SOME SPECIAL MUTUAL CHANGES 79 



9. Demonstrate that dry glass is an insulator, while moist or strongly 

 heated glass is a conductor. In order to do so, arrange a piece of glass 

 so that it forms part of the matter connecting an electrified body with 

 an electroscope. 



10. Two plates are carefully insulated and supported on a wooden 

 base. Connect one with an electroscope and then t lectrif y it. Observe 

 the effects produced in the electroscope when the other plate is brought 

 nearer to, and taken further away from, the plate connected with the 

 electroscope. Frame some hypothesis in explanation of the changes 

 observed. 



53. Electric Phenomena Produced by another Method, 

 If two long strips of zinc and copper be placed, without touch- 

 ing one another, in dilute hydrogen sulphate, and then con- 

 nected with the quadrant electrometer, a movement of the 

 vane will be seen, just as with bodies electrified by friction. 



If the two free ends of the strips are connected with two 

 large plates placed face to face, a light suspended body may 

 be caused to move, when placed between them, just as if these 

 plates had been electrified by friction or induction. In the 

 same way, too, the plates, if removed from the system by 

 non-conducting supports, remain in the same condition for 

 some time, but lose their property completely when made to 

 touch. The space between the two strips or plates outside 

 the liquid is an electric field of the same nature as the last 

 investigated. 



Any arrangement of material by means of which these 

 results may be obtained, is called an electric cell, and there is 

 great variety in the systems capable of being used. Several 

 cells may be combined into a battery, so as to give greater 

 results. 



The Daniell cell consists of a copper plate, placed in a 

 solution of copper sulphate, and a porous vessel containing 

 zinc sulphate and a zinc rod or plate. All these are placed 

 in an outer vessel, and connections are made from the copper 

 and zinc. It will be found that the results, described above 

 as obtainable from a cell, are independent of the size of any 

 portion of the cell, and depend solely on the nature of the 

 materials used. 



