610 ELECTRICAL MEASUREMENTS 



Northrup Alternating- and Direct-current Comparator. 



The indicating portion of the Northrup comparator may be de- 

 scribed as a differential hot-wire millivoltmeter (or milliammeter) . 

 The general features of its construction will be evident from 

 Fig. 372. The "hot wires" are shown at a, d, c, and a', d', c'; 

 they are supposed to be of the same diameter, resistance, and 

 coefficient of expansion, and to be placed in similar environments. 

 They are placed in a horizontal position, and shielded from all 

 drafts by a suitable case. 



FIG. 372. Diagram for Northrup comparator. 



At the middle of their lengths they are connected by an insu- 

 lating bridge piece, which carries a mirror and is drawn down- 

 ward by a spring, so that both wires are taut. A telescope and 

 scale are used to observe the angular deflection of the mirror. 

 In the ideal case, if the same current flows through the two 

 wires, they heat and therefore expand equally. In this case the 

 mirror moves back a little without being tilted, and the scale 

 reading is unchanged. If, however, the currents are not the 

 same, the wires are heated and expand unequally, and a deflec- 

 tion will be observed, which may be reduced to zero by altering 

 one of the currents. When the instrument is in use one wire is 

 traversed by direct, the other by alternating current, and a 

 means is thus afforded of telling when the currents are of equal 

 strengths. After the currents have been adjusted to equality, 

 the direct current is measured by any convenient and accurate 

 means. With the appropriate auxiliary devices, the com- 

 parator may be used for the calibration of alternating-current 

 ammeters and voltmeters. 



Wattmeters. When two wattmeters are to be compared, the 

 current coils are placed in series and the potential coils in parallel. 

 If both instruments are of the dynamometer type direct current 

 may be used, reversals being taken to eliminate the effects of 

 the local field. 



