426 PROBLEMS AND EXERCISES. 



to be '00374 electrostatic units (C.G.S. system). The ratio of 

 the electrostatic to the electromagnetic unit of potential is given 



in Art. 365, being = -. The volt is denned as io 8 electromag- 

 netic units. From these data calculate the E. M. F. of a 

 DanielPs cell in volts. Ans. 1-115 vo ^' 



24. The radius of the earth is approximately 63 x io 7 centi- 

 metres. The ratio of the electrostatic to the electromagnetic 

 unit of capacity is given in Art. 365. The definition of the 

 farad is given in Art. 328. Calculate the capacity of the earth 



(regarded as a sphere) in microfarads. 



Ans. 700 microfarads (nearly). 



25. The electromotive-force of a Darnell's cell was determined 

 by the following process : Five newly-prepared cells were set 

 up in series with a tangent galvanometer, whose constants were 

 found by measurement. The resistances of the circuit were also 

 measured, and found to be in total 16*9 ohms. Knowing the 

 resistance and the absolute strength of current the E. M. F. could 

 be calculated. The deflection obtained was 45, the number of 

 turns of wire in the coil io, the average radius of the coils u 

 centimetres, and the value of the horizontal component of the 

 earth's magnetism at the place was O'i8 C.G.S. units. Deduce 

 the E.M.F. of a Daniell's cell. 



Ans. i -0647 x io 8 C.G.S. units, or 1-0647 volt. 



QUESTIONS ON CHAPTER VII. 



1. I have seen a small chain in which the alternate links 

 were of platinum and silver wires. When an electric current 

 was sent through the chain the platinum links grew red hot 

 while the silver links remained cold. Why was this ? 



2. Calculate by Joule's law the number of heat units developed 

 in a wire whose resistance is 4 ohms when a steady current of 

 14 weber is passed through it for ten minutes. 



% Ans. 1 1 -2 units of heat. 



3. What sort of cells ought to be the best for providing 

 currents to fire torpedo shots ? 



4. Explain why a regulator like that of Duboscq is employed 

 in obtaining a steady voltaic arc. 



