A POCKET GALVANOMETER. 



301 



Of course, we cannot regard this coil as equivalent to a single 

 magnetic shell of an infinite extent. But referring to the diagram of 

 the field of force given in Fig. 6, and the table of page 293 we see 

 that the variation of the action of the coil due to deflections of the 

 magnet will be insensible, if the height of the coil be such that the 

 part outside the jaw is just below the base of the instrument, and if 

 the length of the coil be such that the bends are distant from the 

 nearest poles of the magnets by more than five times the distance 

 between the magnets. 



Further we see that the part of the coil outside the jaw will add 

 to the effect of the part inside by about 33 per cent the current 

 necessarily being in oj)posite directions in these two parts. 



The coil shown in Fig. 12. has 600 turns of copper wire and a 

 resistance of 100 ohms. 



Comparison with a standard galvanometer gave the following 

 results. 



Standard Galvanometer. 



. Pocket Galvanometer. 



Eeadiug. 



Reduced to 

 Amperes. 



ReaJiug. 



Value of 1 division 

 in Ampères. 



11.8 



17.4 

 21.2 



.03446 

 .05081 

 .0619 J 



41.2 



61.0 

 74.5 



.000836 

 .00C833 

 .00C831 



Mean .00(^833 



Hence when au extra resistance of 5000 ohms is added 1 division 

 of the scale will correspond to 4.25 vohs (= .000833 x 5100). 



Thus we see that the present instrument measures from .001 

 ampères to 1G8 Ampères and from 4 volts to 400 volts with a 

 probable error of 1 per cent. The galvanometer has of course three 

 constants, one for the jaw, one for the internal coil, and one for the 



