PROBLEMS AND EXERCISES. 413 



2. Six magnetised sewing-needles are thrust vertically through 

 six little floats of cork, and are placed in a basin of water with 

 their N. -pointing poles upwards. How will they affect one 

 another, and what will be the effect of holding over them the 

 S. -pointing pole of a magnet ? 



3. What distinction do you draw between magnets and 

 magnetic matter ? 



4. On board an iron ship which is laying a submarine tele- 

 graph cable there is a galvanometer used for testing the continuity 

 of the cable. It is necessary to screen the magnetised needle of 

 the galvanometer from being affected by the magnetism of the 

 ship. How can this be done ? 



5. How would you prove two magnets to be of equal 

 strength ? 



6. The force which a magnet-pole exerts upon another 

 magnet-pole decreases as you increase the distance between 

 them. What is the exact law of the magnetic force, an4 how 

 is it proved experimentally ? 



7. What force does a magnet-pole, the strength of which is 

 9 units, exert upon a pole whose strength is 16 units placed 

 6 centimetres away ? Am. 4 dynes. 



8. A pole of strength 40 units acts with a force of 32 dynes 

 upon another pole 5 centimetres away. What is the strength 

 of that pole ? Ans. 20 units. 



9. It is desired to compare the magnetic force at a point 10 

 centimetres from the pole of a magnet with the magnetic force 

 at 5 centimetres' distance. Describe four ways of doing this. 



I o. Explain the phenomenon of Consequent Poles. 



11. In what direction do the lines of magnetic induction (or 

 "lines of force") run in a plane in which there is a single 

 magnetic pole ? How would you arrange an experiment by which 

 to test your answer? 



12. W T hat is a Magnetic Skein What is the law of the 

 potential due to a magnetic shell ? 



13. A steel bar magnet suspended horizontally, and set to 

 oscillate at Bristol, made no complete oscillations in five 



