SOME SPECIAL MUTUAL CHANGES 73 



body. The smaller body may be brought to this condition 

 either by friction or by being placed in contact with an elec- 

 trified body. The same effect will be produced in each case. 

 That is, it is now in a condition such that it may be either 

 attracted or repelled, according to circumstances, whereas a 

 body which is not electrified is never repelled by an electrical 

 body. 



It will be seen that the behaviour just described explains 

 why a body, which is not electrified, may be first attracted and 

 then repelled by an electrified body. 



A and B in each of the above diagrams are mutually 

 electrified bodies, and the condition of the electric field is in- 

 dicated by the behaviour of the bodies placed in it. 



Additional Exercises and Questions. 



1. Perform experiments with the view of finding out whether elec- 

 trification is a property of matter or of space. Show, for example, that 

 the space near an electrified body differs from other space. 



2. Instead of saying that the condition of space is altered, we might 

 say that an electrified body acts at a distance. Discuss this theory. 



3. How would you investigate experimentally whether two bodies 

 are in the same electric condition, and how would you ascertain if they 

 are in equal electric conditions that is, if they are equivalent in causing 

 those changes which are characteristic of electrified bodies ? How would 

 your results be explained if you attribute the changes in question to 

 a condition of space rather than to the condition of the material bodies 

 engaged ? 



4. Do you consider the terms ' positive ' and ' negative,' as applied to 

 electrification, suitable ? 



5. Perform several experiments which show that electrification is a 

 two-sided phenomenon. 



6. Explain what is meant when it is stated that there are ' two 

 kinds of electricity.' 



50. The Existence of Electric Stress indicated by the Elec- 

 troscope. Two light bodies, for convenience, strips of gold 

 leaf, are suspended side by side from a metal rod, and enclosed 

 within a glass vessel for protection from currents of air. The 

 metal rod may terminate either in a disc or a sphere. "When 



