OSCILLOGRAPHS 



73 



Hi-Ids, giving rise to a torque which will rotate the movable system 

 through an angle sufficient to restore coincidence of fields. Corre- 

 sponding, therefore, to each value of the phase difference between 

 i In- currents in the two systems, there is a perfectly definite position 

 nt i-i[uilibrium for the movable system. The value of the phase 

 dilliTence 0, or of the corresponding power factor cos ^, is read oft' 

 on a scale over which moves a pointer attached to the movable 

 ^y -inn. The two systems of coils are connected up like the circuits 

 of three wattmeters, the current coils being placed in series with 

 the three line wires, and the three star-connected pressure coils being 

 joined in series with three large non-inductive resistances, and then 

 connected to the line wires. If the three-phase circuit on which the 

 instrument is used is unbalanced, the reading corresponds to the mean 

 value of the power factor for the three phases. 



38. Oscillographs 



In many cases, it is important to know the exact shape of the 

 p.d. or current wave. Numerous methods of determining the wave- 

 shape are available, but by far the quickest and most convenient is 

 that involving the use of a special instrument devised for this purpose, 

 and known as an oscillograph. We shall select for description a 

 form of oscillograph due to Blondel. 



Every oscillograph is in reality a galvanometer whose moving 

 system is capable of extremely rapid vibration, and which is fitted 



cnmotiCM. UNS 



ARC LI6.HT 



ROLLER TO FIT CAM-PlATC 

 Of MOTOR. 



FIG. 55. Arrangement for recording Vibrations of Oscillograph. 



with a suitable arrangement for recording the vibrations. This 

 arrangement is shown diagrammatically in Fig. 55, and is, except as 

 regards details of construction, the same in all forms of oscillograph. 

 Light coming from an arc passes through a cylindrical lens, and falls 

 on the tiny mirror attached to the moving part of the oscillograph 

 galvanometer. The vibrations of the mirror cause the reflected beam 



