558 ELECTRICAL MEASUREMENTS 



tip is very hard, takes a high polish and does not corrode. The 

 pen friction is thus reduced. The pen is carried by a jointed 

 arm attached to the movable stem. The two members of the 

 arm are nearly perpendicular and the pen is pressed against the 

 paper by a small weight attached to a bell-crank lever carried 

 by the first member; the unweighted arm of the bell crank is 

 attached to the second member by a light cord. 



FIG. 341. Pen and ink reservoir for General Electric Co. curve-drawing 



meters. 



Relay Instruments. The essential features of a registering 

 relay wattmeter for three-phase circuits are shown in Fig. 342. 



The two dynamometer elements are shown at 8 and 9. The 

 spindle carries the outer end of the flat spiral spring, 13, the inner 

 end of which, 15, is attached to an arbor which is turned by the 

 motor 18. The motor is controlled by a circuit through the relay 

 points, 22, 23, 24, so that the torque on the coil is kept balanced 

 by that of the spring. The excursion of the pen on the chart is 

 proportional to the twist in the spring and the resulting diagram 

 is on rectangular coordinates. 



A relay voltmeter is shown in Fig. 343. The six coils are ar- 

 ranged as in the Kelvin balance. The power for moving the 

 pen is obtained from the auxiliary source B by means of the 

 solenoids P and PI, one of which moves the pen to the right, the 

 other to the left. The controlling spring S connects the link M 

 and the balance arm, the latter being provided with a contact 

 finger which plays between the contacts at D. On the passage 

 of the current the finger is brought in contact with the lower stop, 

 thus energizing the solenoid P and moving the pen to the right 

 until the tension on S is sufficient to break the contact; P then 

 becomes inactive, the linkage tends to return to the zero position 



