THE MEASUREMENT OF CURRENT 91 



of currents in special investigations such as are necessary in the 

 determination of electrical standards it is of great service, and is 

 generally considered to be the most accurate device which has 

 been developed for the purpose. Its advantages are: 



1. The constant of the instrument depends principally upon 

 the ratio of the effective radii of the coils. This number can be 

 determined experimentally to a high degree of accuracy, by a 

 method originally due to Bosscha; the difficulties met with in 

 determining the mean radii of multiple-layer coils from mechan- 

 ical measurements are thus avoided. This is the peculiar ad- 

 vantage of the Rayleigh form of balance. 



2. The measurements are independent of the local field and 

 its variations. 



3. The determination of torsion constants is entirely avoided. 



4. Analysis shows that when the distance between the fixed 

 and movable coils is equal to one-half the radius of the larger 

 coil, slight inaccuracies in the placing of the movable coil produce 

 very small errors in the calculated constant of the instrument. 



Fig. 48 shows the current balance used by Rosa, Dorsey and 

 Miller, together with an enlarged view of the coil system. The 

 two fixed coils have a radius of 50 cm. and are placed 25 cm. 

 apart. They are wound on brass frames, the material for which 

 was carefully selected, for even good brass is slightly magnetic. 

 Enamelled wire was used. The movable coil, 25 cm. in diameter, 

 is hung from a precision balance and the force determined by the 

 change in weight necessary to restore the balance to equilibrium 

 when the current in the fixed coil is reversed. This change was 

 6 gm. To prevent disturbance of the balance by air currents 

 set up by the heating of the coils, the fixed coils are water- 

 cooled and the movable coil is hung in a water-jacketed cham- 

 ber which is kept at a constant temperature. 



When the balance is used in the manner indicated, 



M is the change in the weights, corrected for buoyancy of 

 the air, which is necessary to restore the balance to equilibrium 

 when the current in the fixed coil is reversed, and g is the accelera- 

 tion due to gravity. 



