ALTERNATING-CURRENT INSTRUMENTS 



69 



south pole. An adjacent permanent magnet keeps the reeds 

 polarized, so that the reed of a particular mechanical frequency 

 will respond to the same electrical frequency. The reeds are 

 visually so arranged that there is a reed for every half cycle. 

 Figure 65 shows the Frahm type of indicator, as manufactured 

 by Hartmann and Braun. 



39. Power-factor Indicators. Power-factor indicators and 

 synchroscopes are based on the principle of the Tuma phase- 

 meter. In Fig. 66, F is a fixed coil carrying the circuit current. 

 MM' are two flat coils wound with fine wire. They are fastened 

 rigidly together and mounted on a spindle free to rotate. There 



Load 



Fio. 66. Principle of Tuma phase-meter. 



is no mechanical control whatever of this moving element, such 

 as springs, for example. The angle between the coils is 90, 

 01- nearly so. The windings of the two coils MM' are connected 



beral the common point .l.and.l is connected to the same 

 side of the circuit as /'. A non-inductive resistance /.' is con- 

 i between M and the other side of the line. A hiuh induc- 

 tance L is connected between .17' and the other side of the line. 

 Tin- mi-rents in .!/ and M' may be assumed to differ by '.HI in 

 time-: Assume that the power-factor of the load is unity. 



The current in coil M' lau r s the line volt:: " . hen,, 



the flux due to '-oil /' by 90 and therefore exerts no torque. The 

 current in coil V i- in time-phase with the line voltage and 

 hence with the flux due to coil F and will therefore move into 



