THE MEASUREMENT OF CURRENT 3 



through an angle and take up a position along the resultant of 



Fandtf 



Then 



F 2irn 



or 



IT 



I = - tan 9 in absolute units (1) 



IQH . ,, , 



/ = 7: tan 6 in amperes (la) 



Zwn 



The quantity depends on the dimensions of the instrument 



and is the strength of field at the center of the coil due to unit 

 current. It is frequently called the gal- 

 vanometer constant of the coil and de- y 

 noted by G. 



In this elementary demonstration it has H 

 been assumed that: 



1. The coil is perfectly circular. 



2. The mean radius of the windings 



correctly represents the effective radius. F ^- 2. Fields at 

 ,-,, . , ,, needle of tangent galva- 



3. The needle- is exactly at the center nometer. 



of the coil, and the field acting on a finite 



needle is the same as that at the mathematical center of the 



coil. 



4. The needle is in a uniform field and consequently as it 

 deflects, its poles do not swing into a field of strength differing 

 from that at its zero position. 



5. The plane of the coil is in the magnetic meridian and truly 

 vertical. 



6. The factor H, or the horizontal intensity of the local 

 magnetic field, has been determined at the place occupied by the 

 instrument and is constant. 



7. Only magnetic forces act on the needle; that is, there is no 

 friction and no torsional rigidity in the suspension. 



In a careful study of the instrument it would be necessary to 

 discuss each of these items and to determine the numerical effect 



