PROBLEMS AND EXERCISES. 421 



QUESTIONS ON CHAPTER V. 



1 . Define magnetic potential, and find the (magnetic) potential 

 due to a bar magnet 10 centimetres long, and of strength 80, 

 at a point lying in a line with the magnet poles and 6 centi- 

 metres distant from its N. -seeking end. Ans. 8*6. 



2. A N. -seeking pole and a S. -seeking pole, whose strengths 

 are respectively +120 and 60, are in a plane at a distance 

 of 6 centimetres apart. Find the point between them where 

 the potential is = o ; and through this point draw the curve of 

 zero potential in the plane. 



3. Define "intensity of the magnetic field." A magnet 

 whose strength is 270 is placed in a uniform magnetic field 



whose intensity is *i66. What are the forces which act upon 

 its poles ? Ans. + 45 dynes and 45 dynes. 



4. Define " intensity of magnetisation. " A rectangular bar- 

 magnet, whose length was 9 centimetres, was magnetised until 

 the strength of its poles was 164. It was 2 centimetres broad 

 and *5 centimetre thick. Supposing it to be uniformly magnet- 

 ised throughout its length, what is the intensity of the magnet- 

 isation? Ans. 164. 



e 5. Poisson suggested a two -fluid theory of magnetism, the 

 chief point of the hypothesis being that in the molecules of iron 

 and other magnetic substances there were equal quantities of 

 two opposite kinds of magnetic fluid ; and that in the act of 

 magnetisation the two fluids were separated. What facts does 

 this theory explain ? What facts does it fail to explain ? 



6. A current whose strength in "absolute" electromagnetic 

 units was equal to 0*05 traversed a wire ring of 2 centimetres 

 radius. What was the strength of field at the centre of the 

 ring? What was the potential at a point P opposite the 

 middle of the ring and 4 centimetres distant from the circum- 

 ference of the ring. Ans. f -1571 V = -003125. 



7. What limits are there to the power of an electromagnet ? 



8. What is the advantage of the iron core in an electro- 

 magnet ? 



9. Assuming the effective coefficient of magnetisation of iron 



