CHAPTER III 



ALTERNATING-CURRENT INSTRUMENTS AND 

 MEASUREMENTS 



ELECTRO-DYNAMOMETER TYPE INSTRUMENTS 



27. The Siemens Dynamometer. Several types of alternat- 

 ing-current instruments operate on the electro-dynamometer 

 principle. The Siemens dynamometer, Fig. 48, is an example 

 of this type of instrument in simple form. It consists primarily 

 of two sets of coils. The coil F is fixed and the coil M, whose 

 UDB is at right angles to the axis of 

 F, is free to turn through a small 

 angle. M is suspended by a silk 

 thread and its turning moment is 

 opposed by a helical spring. Current 

 is led into the moving coil through 

 two mercury cups. 



When used as an ammeter, the two 

 c.il< are wound with a few turns of 

 coarse wire and are connect ed in 

 B. When current flows through 

 these coils, there is a t en< lency for t he 

 moving coil to swing into the plane 

 of the fixed coil. When the current 

 reverses, it reverses in the two coils 

 simultaneously BO that the torque is always in the same direc- 

 tion. The movable coil is not allowed to deflect, however, but 

 is kept in it- /em portion by turning the knurled head at the 

 top of the instrument which acts on the coil through the spring. 

 The annle by which it is necessary to turn this head is pro- 

 poi!ion;i! t.. the turning moment of the coil. The turning 

 moment i> proportional to the current squared, so that the.: 



tioi) of the III. le\ 



D = A/ 



\\here A 



Fio. 48. Siemens dyna- 

 mometer. 



