430 PROBLEMS AND EXERCISES. 



9. A compass needle, when set swinging, comes to rest 

 sooner if a plate of copper is placed beneath it than if a plate of 

 glass or wood lies beneath it. Explain this fact. 



10. Explain how it is that on making circuit the current 

 rises only gradually to its full strength, especially if there are 

 large electromagnets in the circuit. 



11. Foucault set the heavy bronze wheel of his gyroscope 

 spinning between the poles of a powerful electromagnet, and 

 found that the wheel grew hot, and stopped. What was the 

 cause of this ? Where did the heat come from ? 



12. The strength of the field between the poles 01 a large 

 electromagnet was determined by the following means : A 

 semi-circular coil, consisting of 40 turns of fine insulated wire, 

 mounted on a handle, was connected to the terminals of a long- 

 coil galvanometer having a heavy needle. On inverting this coil 

 suddenly, at a place where the total intensity of the earth's mag- 

 netic force was '48 unit, a deflection of 6 was shown as the first 

 swing of the galvanometer needle. The sensitiveness of the 

 galvanometer was then reduced to T ^-g- by means of a shunt. The 

 little coil was introduced between the poles of the electromagnet 

 and suddenly inverted, when the first swing of the galvan- 

 ometer needle reached 40. What was the strength of the field 

 between the poles? Ans. 3157 units. 



QUESTIONS ON CHAPTER XI. 



1. It is found that a single Daniell's cell will not electrolyse 

 acidulated water, however big it may be made. It is found, on 

 the other hand, that two Daniell's cells, however small, will 

 suffice to produce continuous electrolysis of acidulated water. 

 How do you account for this ? 



2. W T hen a gramme of zinc combines with oxygen it gives 

 out 1301 heat-units. When .this zinc oxide is dissolved in 

 sulphuric acid 369 more units are evolved. To separate an 

 equivalent amount of copper sulphate into sulphuric acid and 

 copper oxide requires 588 heat -units to be expended. To 

 separate the copper from the oxygen in this oxide requires 293 

 more heat-units. The absolute electro - chemical equivalent of 

 zinc 130*003412 (see Art. 212), and Joule's dynamical equivalent 



