GOO ELECTRICAL MEASUREMENTS 



total resistance increases somewhat. In a 150-volt instrument 

 the net effect is negligible, but lower-range instruments, made 

 by using the same galvanometer element and a smaller series 

 resistance, will be affected by an amount increasing with the 

 diminution of the range, 0.4 per cent per degree being the extreme 

 value, for then the copper of the moving coil becomes relatively 

 more important. Therefore, in accurate work very low-range 

 instruments should be used with care. Frequently, in laboratory 

 voltmeters, a thermometer is inserted in the case as an aid in 

 making the temperature corrections. 



From the standpoint of external temperature effects the shunt 

 and the millivoltmeter in shunted ammeters should be of the same 

 material, so that they may have practically the same tempera- 

 ture coefficient. As the shunt is best made of manganin, this 

 implies that a resistance also of manganin be used in series with 

 the copper moving coil, so as to obtain an approximation to the 

 ideal condition. This means that the drop in precision ammeters 

 is considerable 150 to 200 millivolts at full load. The drop 

 in switchboard shunts is about 50 millivolts. 



Internal Heating Errors. In shunted ammeters, errors may 

 arise from the unequal percentage increase of the resistances of 

 the shunt and the millivoltmeter parts, due to the passage of the 

 current. In old instruments, with internal copper shunts, this 

 error is very pronounced. For instance, in a certain 150-ampere 

 instrument, it was found to be 4 per cent at full-scale deflection. 

 In modern high-resistance precision ammeters, this error ceases 

 to be troublesome. 



In voltmeters, if they are kept in circuit, there will be heating 

 of the series resistance and movable coil due to the passage of the 

 current, but on account of the low net temperature coefficient, 

 the resulting error will not be great. High resistance mul- 

 tipliers should be properly ventilated. 



In direct-current instruments (150-volt) the expenditure of 

 energy is small, about 1.5 watts at full-scale deflection. In 

 wattmeters and alternating-current voltmeters, together with 

 their accompanying multipliers, much more heat must be dissi- 

 pated on account of the lower resistances, about 7 watts in a 

 150-volt instrument at full-scale deflection. In any case the 

 construction should be such that the heat is kept away from the 



OF 



FACULTY OF APPLIED SdENCE. 



