420 



ELECTRICAL MEASUREMENTS 



Deflecting 

 ' Coil 



ter's original method, the resistance r was absent and variable 

 currents were used. 



SOURCES OF CURRENT 



In discussing all of these bridge methods sinusoidal currents 

 have been assumed. If a telephone is to be used as a detector 

 and the bridge settings are to be made with precision it is neces- 

 sary that this assumption be closely fulfilled. As far as the 

 balancing is concerned, when a vibration galvanometer is used 

 the wave form is not so important. There are cases, however, 

 where the balance depends on the frequency, so it is highly desir- 

 able that the source of current be one which naturally gives a 



sinusoidal wave. 



Vreeland Oscillator. As the 

 amount of power required is 

 generally small, one is not limited 

 to the use of a dynamo and for 

 bridge .work the best source of 

 current is the Vreeland Sine 

 Wave Oscillator. 28 



The wave form obtained from 

 the device is accurately sinu- 

 soidal and the frequency, which 

 can be varied from about 160 up 

 to 4,000 cycles per second, is 

 constant for any particular setting of the apparatus. 



The device is shown diagrammatically in Fig. 245 and as 

 actually constructed in Fig. 246. The arrangement is such that 

 sinusoidal oscillations of the desired frequency are set up in the 

 primary of an air-core transformer, the secondary of which is 

 connected to the bridge. 



The large glass bulb G is the container for a mercury vapor arc 

 in vacuo. The mercury cathode is at D and two carbon anodes 

 are at A and B. H is any convenient source of direct current. 

 Ri and #2 are controlling resistances and LI and L 2 are two 

 reactors which tend to prevent rapid changes of the direct current 

 which flows from H . In starting the arc an auxiliary electrode 

 F is used and by closing the switch K and tilting the container 

 G, the mercury in F and that in D are brought into contact. 



FIG. 245. Diagram for Vreeland 

 sine wave oscillator. 



