ADVANCED ELECTRICITY AND MAGNETISM. 



B 



connected electrically in series, and the current / to be meas- 

 ured flows through both coils. The suspended coil B is shown 

 in both figures as a single turn of copper rod, and electrical con- 

 nections are made with B throygh the 

 mercury cups cc. The coil B is sus- 

 pended by a fine thread /. A coiled 

 spring 5 is attached at one end to the 

 suspended coil B (at the point a), and 

 the other end of S is attached (at the 

 point b) to the head h so that by turn- 

 ing h the spring S can be twisted and 

 the angle of twist $ can be read off the 

 divided circle. When the spring 5 is un- 

 twisted and when no current is flowing 

 through A and B, the pointer p stands 

 in a certain zero position. When the 

 current /, to be measured, flows through 



A and B a torque or turning force is exerted on B by A , 

 and this torque is balanced by turning h through an observed 

 angle <f> until the pointer p again stands at its zero position. 

 Then / is proportional to V <, that is : 



B 



Fig. 29. 



I = 



(I) 



in which k is a constant which is called the reduction factor of 

 the instrument. 



The correctness of equation (i) may be made evident as fol- 

 lows: The stationary coil A produces a magnetic field which 

 pushes sidewise on the vertical legs of coil B. If the current / 

 is doubled this field is doubled, and the doubled field acting on 

 doubled current in coil B pushes sidewise on the legs of B 

 with quadrupled force, according to equation (i) of Art. 13; but 

 to balance this quadrupled force a quadrupled twist of the spring 

 S is necessary. Therefore < is proportional to T 2 , or / is 

 proportional to 1/0. 



