INDUCTION INSTRUMENTS 447 



hollow aluminum drum B which forms the movable element. 

 The similarity of this arrangement to that of Ferraris is evident. 



The general explanation of the action of induction ammeters, 

 voltmeters, watt-meters and watt-hour meters may be based 

 on a consideration of the properties of rotating magnetic fields.* 

 In this text, however, the motion of the rotor, or movable 

 member, will be considered to be due to the reciprocal action of 

 one pair of poles on the currents induced by the other pair of 

 poles. 



Referring to Fig. 258, the fields due to coils FF induce currents 

 in the cylindrical aluminum rotor. These induced currents will 

 tend to flow in closed paths perpendicular to the axis of the coil. 



FIG. 259. Time-phase diagram for induction instrument. 



The various current filaments will lag behind the induced e.m.fs. 

 by angles dependent on the resistances and inductances of the 

 current paths in the cylinder. Call 7 their equivalent phase dis- 

 placement. These currents are in a position to be deflected by 

 the fields set up by coils EE. Similarly the currents induced 

 by the flux from EE will be acted upon by the flux from FF. 

 Assuming sinusoidal currents, the time-phase relations of the 

 various quantities are shown in Fig. 259. Let the fluxes set up 

 by FF and EE be <t>i and $2 respectively. 



The angles 0i and 2 are measured from an arbitrary zero line. 

 The time-phase difference between the two fluxes, $1 and <E> 2 , 

 is fo -- 0i. EI is the e.m.f. induced in the cylindrical or disc 

 rotor by the flux $i and E 2 is that due to <J> 2 . Ei sets up induced 



* Such an explanation is given in SOLOMON, "Electricity Meters," 

 p. 111. 



