Meters and other Electrical Instruments. 5 



quite steady source of continuous current by means of twisted 

 flexible leads. In Table II. are shown the values of the 

 current indicated by the instrument when turned into various 

 positions, the first column giving the direction towards which 

 the front was turned in each case. 





Table 11. 





Direction. 



Apparent 

 Amperes. 



Variation from 

 Minimum. 



K . . 



. . 15-00 







W. . . 



, . 15-20 



1-3% 



s. . . 



, . 15-32 



2-1% 



E. . . 



. 15-20 



1-3% 



It will be seen that the extreme variation is 2*1 per cent., 

 w T hich agrees (within the limits of error of the instrument) 

 with the 2*5 per cent, variation deduced from the observed 

 fields. For lower currents the variation is much more, being 

 in the inverse ratio of the current. With the thin coil the 

 field due to the fixed coil is so much stronger that the varia- 

 tion is slight except at the very lowest currents. 



The thick coil had 7 turns and an area of about 46 sq. 

 centim., and the total maximum flux at full load was found 

 to be 1850, giving mean B==40. At full load the power 

 wasted is 6 '2 w r atts and 9*3 watts for the thick and thin coils 

 respectively. 



(2) Kelvin Balance. — The total flux through the central 

 space of the coils was got by winding the search-coil round 

 the supporting pillar, and taking throws by reversing the 

 current. The resultant flux was about 1600 (for full load). 

 By the astatic arrangement of the swinging coils the instru- 

 ment is made independent of the earth's field. The self- 

 inductance is about 0'0016 henry. 



(3) Bifilar Mirror Wattmeter. — This instrument has ranges 

 up to 50 or 100 amperes at 10 volts and upwards. The 

 numbers given refer to the fixed series coil. With direct 

 currents it is clear that precautions have to be taken to elimi- 

 nate the effects of the earth's field. 



(4) D'Arsonval Galvanometer. — This was a ballistic one 

 (made by Paul) with a narrow swinging coil of the Ayrton- 

 Mather type. The B given in the Table is that in the air- 

 gap in the neighbourhood of the moving coil ; it would seem 

 to be sufficiently great to be practically unaffected by the 

 magnetism of the earth. Besides, as it is an instrument for 

 use in a fixed position, it is only the effect of variable ex- 

 ternal fields that need to be taken into account. This point 



