428 PROBLEMS AND EXERCISES. 



2. A long strip of zinc is connected to a galvanometer by 

 iron wires. One junction is kept in ice, the other is plunged 

 into water of a temperature of 5OC. Calculate, from the table 

 given in Art. 381, the electromotive-force which is producing 

 the current. Ans. 690 microvolts. 



3. When heat is evolved at a junction of two metals by the 

 passage of a current, how would you distinguish between the 

 heat due to resistance and the heat due to the Peltier effect ? 



4. Sir W. Thomson discovered that when a current flows 

 through copper it absorbs heat when it flows from a hot point 

 to a cold point ; but that when a current is flowing through 

 iron it absorbs heat when it flows from a cold point to a hot 

 point. From these two facts, and from the general law that 

 energy tends to run down to a minimum, deduce which way a 

 current will flow round a circuit made of two half-rings of iron 

 and copper, one junction of which is heated in hot water and the 

 other cooled in ice. 



QUESTIONS ON CHAPTER IX. 



1. Give the reasons which exist for thinking that light is an 

 electromagnetic phenomenon. 



2. How is the action of magnetic forces upon the direction 

 of the vibrations of light shown? and what is the difference 

 between magnetic and diamagnetic media in respect of their 

 magneto-optic properties ? 



3. It was discovered by Willoughby Smith that the resistance 

 of selenium is less when exposed to light than in the dark. 

 Describe the apparatus you would employ to investigate this 

 phenomenon. How would you proceed to experiment if you 

 wished to ascertain whether the amount of electric effect was 

 proportional to the amount of illumination ? 



QUESTIONS ON CHAPTER X. 



I. The ends of a coil of fine insulated wire are connected 

 with terminals of a long-coil galvanometer. A steel bar-magnet 



