422 PROBLEMS AND EXERCISES. 



to be 20, calculate the strength of the pole of an electromagnet 

 whose coils consist of 50 turns of wire of an average radius of 

 I centimetre, when a current of 2 webers- per -second passes 

 through the coils, the core consisting of a bar 5 centimetres 

 long and of I square centimetre of area in its cross section. 



Ans. 1256-6 units. 



10. Enunciate Maxwell's rule concerning magnetic shells, 

 and from it deduce the laws of parallel and oblique currents 

 discovered by Ampere. 



11. A circular copper dish is joined to the zinc pole of a 

 small battery. Acidulated water is then poured into the dish, 

 and a wire from the carbon pole of the battery dips into the 

 liquid at the middle. A few scraps of cork are thrown in to 

 render any movement of the liquid visible. What will occur 

 when the N. -seeking pole of a strong bar-magnet is held above 

 the dish ? 



12. Roget hung up a spiral of copper wire so that the lower 

 end/z/.tf dipped into a cup of mercury. When a strong current 

 was sent through the spiral it started a continuous dance, the 

 lower end producing bright sparks as it dipped in and out of 

 the mercury. Explain this experiment. 



13. It is believed, though it has not yet been proved, that 

 ozone is more strongly magnetic than oxygen. How could this 

 be put to proof? 



QUESTIONS QN CHAPTER VI. 



1. The resistance of telegraph wire being taken as 13 ohms 

 per mile, and the E. M. F. of a Leclanche cell as 1-5 volt, 

 calculate how many cells are needed to send a current of 12 

 milliwebers through a line 1 20 miles long ; assuming that the 

 instruments in circuit offer as much resistance as 20 miles of 

 wire would do, and that the return-current through earth meets 

 with no appreciable resistance. Ans. 15 cells. 



2. 50 Grove's cells (E. M. F. of a Grove = I -8 volt] are 

 united in series, and the circuit is completed by a wire whose 

 resistance is 1 5 ohms. Supposing the internal resistance of each 

 cell to be 0-3 ohm, calculate the strength of the current. 



Ans. 3 webers. 



