298 



A. TAN AK AT) ATE 



4, we make AW practically vanisli. 



Theoretically there is an indefinite number of solutions of tjie 

 equation (C) obtained by salecting sets of values of rti,a.,, «3, «4 such that 



which is always possible since the curve has a maximum. But to 

 apply SLich a mode of solution practically is dangerous on account of 

 the steep ris 3 of the curve toward the maximum. Any small errors 

 in the values of «i, O2 &c. will give rise to an ultimate error greater 

 than that caused by the neglect of the converging terms when a/b 

 is made great. Hence in practice it is advisable to make a/b > 4, and 

 limit the range of deflection so that the least value of ^/b should not 

 fall within 2 or 3. If the instrument were constructed on a large 

 .scale with air the dimensions determined accurately, the graphical 

 solution just indicated might be applicable. 



Fig. 10. above shows the actual coil belonging to the instrument 

 represented in Fig. 4. which was purposely drawn Avithout the coil 

 so as to lay open its internal construction. Both these figures are 

 drawn very nearly to full size. The central ridges which divide the 

 upper and the lower coils into two parts serve for holding the pivots 

 of the frame of the magnets. 



The fittings of the levers and springs remain, of course, exactly 

 the same as in Fig. 4. 



The coil is wound with a thin copper wire in four layers and 



has a resistance of 40 



Fig. 11. 



Ovlvanometer coil. 



Resistauce coil. 



L. 



ohms. An extra re- 

 sistance of 5000 ohms 

 is added by introduc- 

 ing a small bobbin of 

 fine german silver wire 

 not îrhown in Fig. 1. 



