PROBLEMS AND EXERCISES. 415 



resistance of the batteiy thus grouped, with the electromotive- 

 force and the resistance of a single cell. 



Ans. The E.M.F. of the battery is six times that of 



one cell. The total internal resistance is one and 



a half times that of one cell. 



6. A piece of silk-covered copper wire is coiled round the 

 equator of a model terrestrial globe. Apply Ampere's rule to 

 determine in which direction a current must be sent through the 

 coil in order that the model globe may represent the condition 

 of the earth magnetically. 



Ans. The current must flow across the Atlantic from 



Europe to America, and across the Pacific from 



America toward India ; or, in other words, must 

 flow always from east toward west. 



7. A current of '24 webers- per -second flows through a 

 circular coil of seventy-two turns, the (average) diameter of the 

 coils being 20 centimetres. What is the strength of the 

 magnetic field which the current produces at the centre of the 

 coil ? Ans. i *o8. 



8. Suppose a current passing through the above coil produced 

 a deflection of 35 upon a small magnetic needle placed at its 

 centre (the plane of the coils being in the magnetic meridian), 

 at a place where the horizontal component of the earth's 

 magnetic force is -23 units. Calculate the strength of the 

 current in webers-per-second. (Art. 200.) Ans. 0-035. 



9. The current generated by a dynamo-electric machine was 

 passed through a large ring of stout copper wire, at the centre 

 of which hung a small magnetic needle to serve as a tangent 

 galvanometer. When the steam engine drove the armature of 

 the generator at 450 revolutions per minute the deflection of the 

 needle was 60. When the speed of the engine was increased 

 so as to produce 900 revolutions per minute the deflection was 

 74. Compare the strength of the currents in the two cases. 



Ans. The^current was twice as great as before, for tan 

 74 is almost exactly double of tan 60. 



10. The current from two Grove's cells was passed through 

 a sine - galvanometer to measure its strength. When the con- 

 ducting wires were of stout copper wire the coils had to be 

 turned through 70 before they stood parallel to the needle. 

 But when long thin wires were used as conductors the coils 



