PROBLEMS AND EXERCISES. 429 



is placed slowly into the hollow of the coil, and then withdrawn 

 suddenly. What actions will be observed on the needle of the 

 galvanometer ? 



2. Round the outside of a deep cylindrical jar are coiled two 

 separate pieces of fine silk-covered wirej each consisting of many 

 turns. The ends of one coil are fastened to a battery, those of 

 the other to a sensitive galvanometer. When an iron bar is 

 poked into the jar a momentary current is observed in the 

 galvanometer coils, and when it is drawn out another moment- 

 ary current, but in an opposite direction, is observed. Explain 

 these observations. 



3. A casement window has an iron frame. The aspect is 

 north, the hinges being on the east side. What happens when 

 the window is opened ? 



4. Explain the construction of the induction coil. What 

 are the particular uses of the condenser, the automatic break, 

 and the iron wire core ? 



5. It is desired to measure the strength of the field between 

 the poles of an electromagnet which is excited by a current from 

 a constant source. How could you apply Faraday's discovery 

 of induction currents to this purpose ? 



6. What is meant by the term "extra-currents?" A small 

 battery was joined in circuit with a coil of fine wire and a 

 galvanometer, in which the current was found to produce a 

 steady but small deflection. An unmagnetised iron bar was 

 now plunged into the hollow of the coil and then withdrawn. 

 The galvanometer needle was observed to recede momentarily 

 from its first position, then to return and to swing beyond it 

 with a wider arc than before, and finally to settle down to its 

 original deflection. Explain these actions. 



7. In what respect do dynamo - electric machines differ from . 

 magneto-electric machines ? Where does the magnetism of the 

 field -magnets come from in the former? Where does the 

 dynamical energy of the currents come from in the latter ? 



8. The older magneto - electric machines produced only 

 intermittent currents, and these were usually alternating in 

 direction. By what means do the more modern magneto-electric 

 generators produce currents which are continuous and direct ? 



