92 ELECTRICAL MEASUREMENTS 



The factor K depends principally on the ratio of the radii of 

 the fixed and movable coils. It is equal to -^ where m is the 



mutual inductance of the coils and dx refers to an axial displace- 

 ment of the movable coil, m can be calculated, in the form of a 

 series, from the numbers of turns and the dimensions of the coils. 



Secondary Current Balances. The Kelvin Balance. In the 

 Kelvin balance there are four fixed and two movable coils, the 

 latter being carried at the ends of a balance arm which turns 

 about a horizontal axis, see Fig. 49. Each movable coil is situ- 

 ated between two fixed coils and all six coils are connected in 

 series. In place of a knife edge, the beam is suspended by a large 

 number of filaments of copper wire, forming a sort of stranded rib- 

 bon, which provides a ready means for making the electrical con- 

 nection to the movable coil. Absence of friction, large radiating 

 surface, and cooling by conduction to the frame are the advan- 

 tages attained by this means. 



The center of gravity of the movable system may be adjusted 

 as in an ordinary balance, by means of a weight on a short verti- 

 cal rod immediately above the point of support. 



The bobbins and the base plate are made of slate, thus insuring 

 rigidity and absence of eddy currents. Means for levelling the 

 instrument and for removing the weight of the movable parts 

 from the suspension wires when the instrument is not in use are 

 provided. 



A uniformly graduated bar is attached to the movable system 

 and is supplied with a sliding carriage upon which the weights 

 can be placed in a definite position, small conical pins on the 

 carriage fitting into corresponding holes in the weights. The 

 carriage can be manipulated from without the case by means of 

 cords attached to a self -releasing pendent, which slides on guides 

 attached to the base of the instrument. 



The zero of the graduated bar is at the extreme left hand. If, 

 when no current is flowing in the balance, the sliding weight is 

 set at zero and the corresponding counterpoise placed in the V-- 

 shaped trough attached to the right-hand movable coil, then the 

 pointer attached to the beam should stand at a reference mark 

 upon the short vertical scale attached to the base of the instru- 

 ment. If this is not so, a means of adjustment is provided in a 



