546 ELECTRICAL MEASUREMENTS 



arrangement is that phase oc is connected to o'b r and phase ob 

 to o'c'. 



Suppose the two machines are in synchronism. The vector 

 diagrams for the electromotive forces are shown in Fig. 330. 



Synchronism is indicated when lamp No. 1 is dark and lamps 

 2 and 3 are glowing. If the machines are not in phase, the vec- 

 tors being in the dotted position, lamp No. 1 begins to glow, No. 

 2 grows dimmer and No. 3 grows brighter, so the dark lamp is 

 passed from position No. 1 to position No. 2. If the phase dis- 

 placement is in the other direction, lamp No. 1 begins to glow, 

 No. 2 grows brighter and No. 3 is dimmed, so that the dark lamp 

 is passed from position No. 1 to position No. 3. 



The lamps are arranged on the switchboard at the corners of 

 an equilateral triangle; by noticing whether the order of brilliancy 

 of the lamps proceeds around the triangle in the right-hand or 

 the left-hand direction, one can tell whether the speed of the in- 

 coming machine must be increased or diminished. 



Frequency Meters. Instruments of this class should be inde- 

 pendent of wave form and also of variations of the line voltage. 

 Because of the latter requisite, certain forms of frequency meter 

 are constructed so that the controlling and deflecting moments 

 acting on the movable system both depend on the current through 

 the instrument, that is, on the line voltage. 



Resonating Frequency Meters. Under the usual operating 

 conditions the range of frequencies which must be covered by 

 any form of frequency meter is small, and the normal frequency 

 of the current has a definite fixed value. It thus becomes natural 

 to employ in these instruments the principle of resonance, 

 either electrical or mechanical. 



In the General Electric Company's resonating frequency meter, 

 advantage is taken of the action of circuits containing inductance 

 and capacity in series. When such a circuit is properly adjusted, 

 if the periodicity of the applied P.D. be varied, the maximum 

 value of the current will be sharply defined, provided the energy 

 losses in the circuit be small. Fig. 331 shows the circuits of 

 this particular instrument. The movable element consists of 

 two crossed coils set very nearly at right angles to each other; it 

 is pivoted and free to move, there being no controlling springs. 



For a 60-cycle instrument one main circuit is tuned to reso- 



