PROBLEMS AND EXERCISES. 425 



galvanometer (also of small resistance) through a wire whose 

 resistance was known to be 435 ohms. The same cell gave a 

 deflection of 5 upon the same galvanometer when a wire of 

 unknown resistance was substituted in the circuit. What was 

 the unknown resistance ? Ans. 790 ohms. 



1 8. In a Wheatstone's bridge in which resistances of 10 and 

 i oo ohms respectively were used as the fixed resistances, a wire 

 whose resistance was to be determined was placed : its resist- 

 ance was balanced when the adjustable coils were arranged to 

 throw 281 ohms into circuit. What was its resistance ? 



Ans. 25-1 ohms. 



19. A battery of 5 Leclanche cells was connected in simple 

 circuit with a galvanometer and a box of resistance coils. A 

 deflection of 40 having been obtained by adjustment of the 

 resistances, it was found that the introduction of 150 additional 

 ohms of resistance brought down the deflection to 29. A battery 

 of ten Daniell's cells was then substituted in the circuit and 

 adjusted until the resistance was 40 as before. But this time it 

 was found that 216 ohms had to be added before the deflection 

 was brought down to 29. Taking the E.M.F. of a single 

 Daniell's cell as I '079 volt, calculate that of a single Leclanche 

 cell. Ans. l -499 volt. 



20. How are standard resistance coils wound, and why? 

 What materials are they made of, and why ? 



2 1 . Three very small Daniell's cells gave, with a sine galvano- 

 meter (itself of no appreciable resistance), a reading of 57. On 

 throwing 20 ohms into the circuit the galvanometer reading fell 

 to 25. Calculate the internal resistance of the cells. 



Ans. 6-6 ohms each. 



22. A knot of telegraph cable was plunged in a tub of water 

 and then charged for a minute from a battery of 120 Daniell's 

 cells. The cable was then discharged through a long -coil 

 galvanometer with a needle of slow swing. The first swing 

 was 40. A condenser whose capacity was \ microfarad was 

 then similarly charged and discharged ; but this time the first 

 swing of the needle was only over 14. What was the capacity 

 of the piece of cable ? Ans. 0^934 microfarad. 



23. Using an absolute electrometer, Sir W. Thomson found 

 the difference of potential between the poles of a Daniell's cell 



