422 



ELECTRICAL MEASUREMENTS 



ACB. The arcs are in the field of the deflecting coils in the cir- 

 cuit ACB and these coils are so wound that they still further 

 deflect the arc away from A and toward B. When the condenser 

 is fully charged, the current in the deflecting coils ceases and the 

 arcs begin to return toward their first condition. The condenser 

 discharges and by means of the deflecting coils the arcs are 

 forced toward A. 



The frequency with which the condenser is charged and dis- 

 charged depends upon the inductance and capacity and is given by 



= _!_/JL) 



The variations in frequency are obtained by adjusting the 

 capacity C. The inductance L depends to a certain extent upon 

 the positions of the two deflecting coils. It is, therefore, neces- 

 sary to determine the frequency after the apparatus has been set 

 up. The calibration is readily made. The current from the 

 secondary is led through a telephone and the note obtained 

 compared by the method of beats with that of a standard 

 tuning fork. After this calibration, the frequency corresponding 

 to any other capacity is readily calculated. 



Care must be taken in locating thfc apparatus, which sets up 

 a considerable stray field. When a telephone is used as the 

 detector the apparatus must be so set up that the note which 

 the oscillator emits does not disturb the observer. 



The Microphone Hummer. If it is not necessary to work at 

 a definite frequency, a simple microphone hummer, as shown in 

 Fig. 247, is a convenient source of current. 



Transmitter Beceiver 



To Bridge 



FIG. 247.-^Diagram for simple microphone hummer. 



