4 i4 PROBLEMS AND EXERCISES. 



minutes ; the same needle when set oscillating horizontally at 

 St. Helena executed 112 complete oscillations in four minutes. 

 Compare the horizontal component of the force of the earth's 

 magnetism at Bristol with that at St. Helena. 



Ans. H at Bristol : H at St. Helena : : 484 : 784. 



14. Supposing the dip at Bristol to be 70 and that at St. 

 Helena to be 30, calculate from the data of the preceding 

 question the total force of the earth's magnetism at St. Helena, 

 that at Bristol being taken as '48 unit. Ans. "307. 



[N.B. The student should see Footnote i, on p. 116.] 



15. A small magnetic needle was placed magnetically north 

 of the middle point of a strong bar-magnet which lay (magneti- 

 cally) east and west. When the magnet was ^feet away from 

 the needle the deflexion of the latter was 2 ; when moved up 

 to a distance of 2 feet the deflexion was 1 5 30' ; and when only 

 I foot apart the deflexion 4vas 37. Deduce the law of the total 

 action of one magnet on another. 



1 6. Describe how the daily irregularities of the earth's mag- 

 netism are registered at different stations for comparison. 



QUESTIONS ON CHAPTER III. 



1 . Show that the total of the differences of potential by con- 

 tact in three simple voltaic cells joined in series is three times as 

 great as the difference of potential in one cell, the materials 

 being the same in each. 



2. How can local action and polarisation be prevented in a 

 voltaic cell? 



3. Supposing the length of spark to be proportional to the 

 difference of potential, calculate from the data of Arts. 291 and 

 178 how many DanielPs cells would be required to yield a 

 sufficient difference of potential to produce a spark one mile long 

 through air. Ans. 5153 million cells. 



4. On what does the internal resistance of a battery depend ? 

 Is there any way of diminishing it ? 



5. Twenty -four similar cells are grouped together in four 

 rows of six cells each ; compare the electromotive-force and the 



