SOME SPECIAL MUTUAL CHANGES 91 



the same arrangement of the circuit. In this case the electric 

 stress causes the thermal change. In the previous case the 

 thermal change caused .the electric stress. 



Additional Exercises and Questions. 



1. How do you distinguish air, copper, brass, mercury, solution of 

 copper sulphate in water, alcohol, turpentine, and glass with regard to 

 conduction ? Give experiments which would support your views. 



2. What may be observed when conduction takes place in homo- 

 geneous metals ? 



3. What analogy exists between a current of electricity and a 

 current of liquid ? 



4. Compare thermal conduction with electric conduction in the case 

 of metals. 



5. What units can you suggest in order to measure ' quantity of 

 electricity ' and ' current ' ? 



6. Describe by means of diagrams the stresses in an electroscope and 

 an electrometer, (1st; when they are at rest and not electrified, and 

 (2nd) when they are at rest and electrified. 



7. What experiments show that the space around bodies forming 

 part of an electric circuit differs from ordinary space, and also from the 

 space in the neighbourhood of an electrified body? What further dis- 

 tinctions can be drawn from the facts that conductivity increases with 

 the sectional area, and that the whole of the body in an electric circuit 

 changes in temperature and not merely the outside ? 



8. Coil a long piece of insulated wire into a helix, connect it hori- 

 zontally by its two ends with a small cell which can be made to float in 

 water, and observe that the coil sets itself with its axis nearly north 

 and south, as a freely suspended magnet would do. Observe also that 

 two such floating coils attract and repel just as two magnets free to 

 move would do. The cell may be made of any suitable materials. 



9. Construct a coil of insulated wire containing many turns, connect 

 the ends with a galvanometer placed at some distance, and observe what 

 happens when a bar magnet is moved in the axis of the coil, or when 

 the coil is moved towards or away from the magnet. Note the result of 

 variation in the rate of the movement. 



10. Substitute for the magnet in the last observation (9) another 

 coil in which a current is flowing. We then have a current in one coil 

 producing a current in a neighbouring coil. Observe also that a bar of 

 soft iron placed within the coil possesses the properties of a magnet as 

 long as the circuit is unbroken. It becomes an electro-magnet. Insert 



