ELECTRICITY METERS 



501 



It will be noticed that the customer's load is carried by the 

 jumper, which is put on before the meter is taken out of service, 

 thus avoiding any interruption of the circuit. In this and other 

 cases where jumpers are used, it is essential that they be so applied 

 that the normal field at the armature of the meter is not disturbed. 



For tests of alternating-current meters after installation, it is 

 possible by the use of a special step-down transformer connected 

 across the mains, to obtain large fictitious loads. This implies 

 that the controlling devices may be made simple and compact 

 and the whole apparatus portable. Such devices are on the 

 market and are sold under the name of phantom load boxes. 

 It is to be remembered that the percentage accuracy of an alter- 

 nating-current meter depends on the power factor of its load, 

 so it is necessary to be sure that the transformer arrangement 

 does not introduce complications due to phase displacements. 



Phase -shifting Devices. In testing and adjusting alternating- 

 current meters in the laboratory, one must be able to vary the 



FIG. 288. Diagram for phase-shifting motor-generator set. 



effective power factor of the load, preferably without any at- 

 tendant alteration in either the current, voltage or wave form. 

 An arrangement for this purpose is shown diagrammatically in 

 Fig. 288. 



It consists of two motor-driven machines, the armatures of 

 which are rigidly coupled; one field is stationary while the other 

 is so mounted that it can be displaced about the axis of the shaft 

 by a wormwheel and sector and its angular position read on 

 a graduated arc. The displacement may be effected by a remote- 



