214 ELECTRICAL MEASUREMENTS 



The coils BB' are in series and together with a suitable re- 

 sistance are connected across the terminals of the magneto D. 

 The coil A is so connected that it is traversed by any current 

 which flows through the specimen when the latter is joined be- 

 tween the external terminals. If the external circuit is open, 

 the movable element, under the action of the current through R, 

 will take up such a position that the plane of the coils BB' coincides 

 with the dotted line. This position corresponds to an infinite 

 external resistance and is marked infinity on the scale. If there 

 be a current in the external circuit, due to the imperfect insulation 

 resistance of the specimen, a current will flow through the coil 

 A and in such a direction that it turns the movable system 

 toward the right, carrying the coils BB' with it until the latter, 

 which move in a non-uniform field, are in a field so strong that 

 the turning moments due to A and BB' are balanced. The 

 coil A may thus be considered to furnish the directive moment 

 which acts on the system. 



The indications are independent of the voltage of the magneto, 

 for if that changes both the deflective and directive moments are 

 altered in the same ratio. 



In one design of the instrument, the magneto is driven through 

 a clutch arrangement controlled by a centrifugal governor so that 

 the voltage cannot rise above a definite maximum; then, when 

 the crank is turned fast enough, a constant testing voltage is 

 obtained. This is of importance when apparatus having a 

 considerable electrostatic capacity is tested. 



MEASUREMENT OF INSULATION RESISTANCES OF COMMER- 

 CIAL CIRCUITS WHEN POWER IS ON 



It is sometimes necessary to measure the insulation resistance 

 to " ground" (the water and gas pipes) of a distribution system, 

 for instance, that of an office building, where the conditions are 

 such that the power is on and the supply must not be interrupted. 

 This case is illustrated by Fig. 122. 



Voltmeter Method. The resistances to ground will be repre- 

 sented by x l and x 2 . 



If the insulation resistances are not above 1 or 2 megohms, 

 recourse may be had to the voltmeter method (page 208). Three 

 voltage measurements suffice to determine both x l and x 2 . 



