ELECTRICAL MEASUREMENT 81 



magnetic oscillograph. By this means as described in the paper 

 already mentioned, it is possible to obtain the curve of variation of 

 inductance and effective resistance of the coil over one low-frequency 

 cycle. 



Another method used at the present time employs the same bridge 

 circuit but an entirely different method of detecting the cyclic variation 

 in the balance. This method of detection uses the cathode-ray oscillo- 

 graph and is as follows. The low-frequency source is connected across 

 a high resistance and condenser in series, the two having equal im- 

 pedances. The potentials across the condenser and resistance are then 

 placed respectively across the horizontal and vertical plates of the 

 oscillograph. These two potentials, being equal in magnitude but 

 90 deg. apart in phase, give a circle on the screen. The output of the 

 main bridge is now connected through a transformer whose secondary 

 is connected in series with the oscillograph cathode potential. Due to 

 the fact that the sensitivity of the tube to deflections by the plate 

 potentials varies with the cathode potential, the radius of this circle 

 produced by the low frequency is a function of the telephone frequency 

 input from the bridge, and instead of a circle we get a band, the width 

 of which is a measure of the degree of unbalance of the bridge. The 

 point in the cycle at which the bridge is balanced, is indicated on the 

 screen as the point where this band diminishes to a line, and the angular 

 position of this point in the band determines the phase position of this 

 balance with respect to the low-frequency cycle. It is possible in this 

 way to balance the bridge for any angular position corresponding to 

 any point in the low-frequency cycle, and by taking sufficient points, 

 to obtain a curve of variation of the coil constants over a complete 

 cycle. This method is found to be simpler and faster than the method 

 using the mechanical oscillograph. 



Inductance Balance. A simple form of bridge for measuring induc- 

 tance balance of the two windings of a transformer or other coil uses 

 the two windings of the transformer for two arms, the other two arms 

 being resistances, one of which at least is variable. The balance is 

 made by means of the variable resistance, the ratio of the two resist- 

 ances at balance then giving the unbalance of the transformer. If one 

 of these resistances is made 100 ohms, the variation of the other 

 from 100 ohms at balance gives directly the percentage unbalance. 

 Any unbalance in resistance is usually comparatively small and may 

 be taken care of by low resistances in series with the transformer 

 windings. 



Ratio of Transformation. A similar bridge may be used for the 

 measurement of ratio of transformation. There are many cases where 

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