00 



ELECTRICAL MEASUREMENTS 



tory work, for the shunted instrument may be calibrated with 

 direct and used with alternating currents. 



To increase the current capacity of this form of indicator the 

 wire AB may be sectionalized, as indicated in Fig. 25. 

 The connections to AB at 1, 2, 3, are made with thin strips of 



silver foil so that the motion is 

 p not impeded. 



The current capacity of the 

 s indicator is about 5 amp., the 

 resistance about 0.05 ohm, 

 consequently the full-load drop 



FIG. 



is approximately 250 milli- 

 volts, or about three or four 

 times that in the ordinary 

 direct-current switchboard shunt. 



As the resistance is so low, it is essential that all connections 

 between the indicator and the shunt be carefully made and that 

 the same set of leads connecting the indicator and the shunt 

 be used during calibration and subsequent use of the ammeter. 



The Duddell Thermo-ammeter. The working parts of this 

 modification of the thermo-galvanometer (see page 46) are 



Zero Set 



and Jewel 



Heater 



Lower Pivot and Guide Jewel 



ermo Junction 

 Receiving Plate 



FIG. 26. Diagram for Duddell thermo-ammeter. 



shown in Fig. 26. The movable coil is so mounted that it is 

 practically supported from the upper pivot, the lower pivot act- 

 ing largely as a guide. Pivot friction is thus minimized. 



The instrument is primarily designed for measurements at 



